Stereospecific nickel-catalyzed cross-coupling reactions of alkyl ethers: enantioselective synthesis of diarylethanes.

Science.gov (United States)

Taylor, Buck L H; Swift, Elizabeth C; Waetzig, Joshua D; Jarvo, Elizabeth R

2011-01-26

Secondary benzylic ethers undergo stereospecific substitution reactions with Grignard reagents in the presence of nickel catalysts. Reactions proceed with inversion of configuration and high stereochemical fidelity. This reaction allows for facile enantioselective synthesis of biologically active diarylethanes from readily available optically enriched carbinols.

Catalytic Enantioselective Synthesis of 3,4-Unsubstituted Thiochromenes through Sulfa-Michael/Julia-Kocienski Olefination Cascade Reaction.

Science.gov (United States)

Simlandy, Amit Kumar; Mukherjee, Santanu

2017-05-05

A highly enantioselective cascade sulfa-Michael/Julia-Kocienski olefination reaction between 2-mercaptobenzaldehydes and β-substituted vinyl PT-sulfones has been realized for the synthesis of 3,4-unsubstituted 2H-thiochromenes. This reaction, catalyzed by diphenylprolinol TMS ether, proceeds through an aromatic iminium intermediate and furnishes a wide range of 2-substiuted 2H-thiochromenes with excellent enantioselectivities (up to 99:1 er).

Catalytic enantioselective alkene aminohalogenation/cyclization involving atom transfer.

Science.gov (United States)

Bovino, Michael T; Chemler, Sherry R

2012-04-16

Problem solved: the title reaction was used for the synthesis of chiral 2-bromo, chloro, and iodomethyl indolines and 2-iodomethyl pyrrolidines. Stereocenter formation is believed to occur by enantioselective cis aminocupration and C-X bond formation is believed to occur by atom transfer. The ultility of the products as versatile synthetic intermediates was demonstrated, as was a radical cascade cyclization sequence. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Asymmetric NHC-catalyzed aza-Diels-Alder reactions: Highly enantioselective route to α-amino acid derivatives and DFT calculations

KAUST Repository

Yang, Limin; Wang, Fei; Lee, Richmond; Lv, Yunbo; Huang, Kuo-Wei; Zhong, Guofu

2014-01-01

A facile N-heterocyclic carbene catalytic enantioselective aza-Diels-Alder reaction of oxodiazenes with α-chloroaldehydes as dienophile precursors is reported, with excellent enantioselectivity (ee > 99%) and excellent yield (up to 93%). DFT study

Enantioselective rhodium/ruthenium photoredox catalysis en route to chiral 1,2-aminoalcohols.

Science.gov (United States)

Ma, Jiajia; Harms, Klaus; Meggers, Eric

2016-08-09

A rhodium-based chiral Lewis acid catalyst combined with [Ru(bpy)3](PF6)2 as a photoredox sensitizer allows for the visible-light-activated redox coupling of α-silylamines with 2-acyl imidazoles to afford, after desilylation, 1,2-amino-alcohols in yields of 69-88% and with high enantioselectivity (54-99% ee). The reaction is proposed to proceed via an electron exchange between the α-silylamine (electron donor) and the rhodium-chelated 2-acyl imidazole (electron acceptor), followed by a stereocontrolled radical-radical reaction. Substrate scope and control experiments reveal that the trimethylsilyl group plays a crucial role in this reductive umpolung of the carbonyl group.

L-Threonine-derived novel bifunctional phosphine-sulfonamide catalyst-promoted enantioselective aza-morita-Baylis-Hillman reaction

KAUST Repository

Zhong, Fangrui

2011-03-18

A series of novel bifunctional phosphine-sulfonamide organic catalysts were designed and readily prepared from natural amino acids, and they were utilized to promote enantioselective aza-Morita-Baylis-Hillman (MBH) reactions. l-Threonine-derived phosphine-sulfonamide 9b was found to be the most efficient catalyst, affording the desired aza-MBH adducts in high yields and with excellent enantioselectivities. © 2011 American Chemical Society.

Regio-, Diastereo-, and Enantioselective Nitroso-Diels-Alder Reaction of 1,3-Diene-1-carbamates Catalyzed by Chiral Phosphoric Acids.

Science.gov (United States)

Pous, Jonathan; Courant, Thibaut; Bernadat, Guillaume; Iorga, Bogdan I; Blanchard, Florent; Masson, Géraldine

2015-09-23

Chiral phosphoric acid-catalyzed asymmetric nitroso-Diels-Alder reaction of nitrosoarenes with carbamate-dienes afforded cis-3,6-disubstituted dihydro-1,2-oxazines in high yields with excellent regio-, diastereo-, and enantioselectivities. Interestingly, we observed that the catalyst is able not only to control the enantioselectivity but also to reverse the regioselectivity of the noncatalyzed nitroso-Diels-Alder reaction. The regiochemistry reversal and asynchronous concerted mechanism were confirmed by DFT calculations.

Enantioselective organocatalyzed Oxa-Michael-Aldol cascade reactions: Construction of chiral 4H-chromenes with a trifluoromethylated tetrasubstituted carbon stereocenter

KAUST Repository

Zhang, Jing

2015-03-13

The first organocatalytic asymmetric synthesis of 4H-chromenes bearing a trifluoromethylated tetrasubstituted carbon center is presented. Chiral secondary amines promote the oxa-Michael-aldol cascade reaction between alkynals and 2-trifluoroacetylphenols via iminium-allenamine activation to produce pharmaceutically important heterocycles with excellent enantioselectivities. The proposed reaction can be scaled-up easily with maintenance of the excellent enantioselectivity. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Manipulating radicals: Using cobalt to steer radical reactions

OpenAIRE

Chirilă, A.

2017-01-01

This thesis describes research aimed at understanding and exploiting metallo-radical reactivity and explores reactions mediated by square planar, low-spin cobalt(II) complexes. A primary goal was to uncover novel reactivity of discrete cobalt(III)-bound carbene radicals generated upon reaction of the cobalt(II) catalysts with carbene precursors. Another important goal was to replace cobalt(II)-porphyrin catalysts with cheaper and easier to prepare metallo-radical analogues. Therefore the cata...

The origin of enantioselectivity in the l-threonine-derived phosphine-sulfonamide catalyzed aza-Morita-Baylis-Hillman reaction: Effects of the intramolecular hydrogen bonding

KAUST Repository

Lee, Richmond

2013-01-01

l-Threonine-derived phosphine-sulfonamide 4 was identified as the most efficient catalyst to promote enantioselective aza-Morita-Baylis-Hillman (MBH) reactions, affording the desired aza-MBH adducts with excellent enantioselectivities. Density functional theory (DFT) studies were carried out to elucidate the origin of the observed enantioselectivity. The importance of the intramolecular N-H⋯O hydrogen-bonding interaction between the sulfonamide and enolate groups was identified to be crucial in inducing a high degree of stereochemical control in both the enolate addition to imine and the subsequent proton transfer step, affording aza-MBH reactions with excellent enantioselectivity. © 2013 The Royal Society of Chemistry.

Asymmetric Construction of Benzindoloquinolizidine: Application of An Organocatalytic Enantioselective Conjugate Addition-Cyclization Cascade Reaction

International Nuclear Information System (INIS)

Kim, Cheolwoong; Seo, Seung Woo; Lee, Yona; Kim, Sunggon

2014-01-01

We have developed the synthetic methodology of enantioenriched benzindoloquinolizidines based on the organocatalytic enantioselective conjugate addition-cyclization cascade reaction of o-N-(3-indoleacetyl)amino-cinnamaldehydes with malonates followed by an acid-catalyzed intramolecular Pictet-Spengler type cyclization. The asymmetric reaction using diphenylprolinol TMS ether as an organocatalyst produces the desired products with good to excellent yields and high enantioselectivities (up to 98% ee). The evaluation of the applications of this synthetic methodology for generating enantioenriched benzindolo-quinolizidines and studies on the biological activity of these compounds against human prostate cancer in particular are now in progress. Results of these studies will be presented in due course. Many new types of chemical reactions have been developed to facilitate easier synthesis of complex compounds. Among the strategies, domino reactions, which have been utilized for the efficient and stereoselective construction of complex molecules from simple precursors in a single process, are widely used due to their high synthetic efficiency by reducing both the number of synthetic operation required and the quantities of chemicals and solvents used

Diastereoselective and enantioselective conjugate addition reactions utilizing α,β-unsaturated amides and lactams

Directory of Open Access Journals (Sweden)

Katherine M. Byrd

2015-04-01

Full Text Available The conjugate addition reaction has been a useful tool in the formation of carbon–carbon bonds. The utility of this reaction has been demonstrated in the synthesis of many natural products, materials, and pharmacological agents. In the last three decades, there has been a significant increase in the development of asymmetric variants of this reaction. Unfortunately, conjugate addition reactions using α,β-unsaturated amides and lactams remain underdeveloped due to their inherently low reactivity. This review highlights the work that has been done on both diastereoselective and enantioselective conjugate addition reactions utilizing α,β-unsaturated amides and lactams.

Cyclic aldimines as superior electrophiles for Cu-catalyzed decarboxylative Mannich reaction of β-ketoacids with a broad scope and high enantioselectivity.

Science.gov (United States)

Zhang, Heng-Xia; Nie, Jing; Cai, Hua; Ma, Jun-An

2014-05-02

A novel Cu-catalyzed enantioselective decarboxylative Mannich reaction of cyclic aldimines with β-ketoacids is described. The cyclic structure of these aldimines, in which the C═N bond is constrained in the Z geometry, appears to be important, allowing Mannich condensation to proceed in high yields with excellent enantioselectivities. A chiral chroman-4-amine was synthesized from the decarboxylative Mannich product in several steps without loss of enantioselectivity.

Bicyclic Guanidine Catalyzed Asymmetric Tandem Isomerization Intramolecular-Diels-Alder Reaction: The First Catalytic Enantioselective Total Synthesis of (+)-alpha-Yohimbine.

Science.gov (United States)

Feng, Wei; Jiang, Danfeng; Kee, Choon-Wee; Liu, Hongjun; Tan, Choon-Hong

2016-02-04

Hydroisoquinoline derivatives were prepared in moderate to good enantioselectivities via a bicyclic guanidine-catalyzed tandem isomerization intramolecular-Diels-Alder (IMDA) reaction of alkynes. With this synthetic method, the first enantioselective synthesis of (+)-alpha-yohimbine was completed in 9 steps from the IMDA products. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Organocatalytic Michael and Friedel–Crafts reactions in enantioselective synthesis of biologically active compounds

International Nuclear Information System (INIS)

Maltsev, O V; Beletskaya, Irina P; Zlotin, Sergei G

2011-01-01

Recent applications of organocatalytic Michael and Friedel–Crafts reactions in enantioselective synthesis of biologically active compounds: natural products, pharmaceutical agents and plant protection agents are reviewed. The key mechanisms of stereoinduction, types of organocatalysts and reagents used in these reactions are considered. The material is classified according to the type of newly formed bonds incorporating the asymmetric carbon atom, and the information for the most numerous C–C coupling reactions is systematized according to the natures of the electrophile and the nucleophile. The bibliography includes 433 references.

Asymmetric NHC-catalyzed aza-Diels-Alder reactions: Highly enantioselective route to α-amino acid derivatives and DFT calculations

KAUST Repository

Yang, Limin

2014-08-01

A facile N-heterocyclic carbene catalytic enantioselective aza-Diels-Alder reaction of oxodiazenes with α-chloroaldehydes as dienophile precursors is reported, with excellent enantioselectivity (ee > 99%) and excellent yield (up to 93%). DFT study showed that cis-TSa, formed from a top face approach of oxodiazene to cis-IIa, is the most favorable transition state and is consistent with the experimental observations. © 2014 American Chemical Society.

Total syntheses of mitragynine, paynantheine and speciogynine via an enantioselective thiourea-catalysed Pictet-Spengler reaction

NARCIS (Netherlands)

Kerschgens, I. P.; Claveau, E.; Wanner, M.J.; Ingemann, S.; van Maarseveen, J.H.; Hiemstra, H.

2012-01-01

The pharmacologically interesting indole alkaloids (-)-mitragynine, (+)-paynantheine and (+)-speciogynine were synthesised in nine steps from 4-methoxytryptamine by a route featuring (i) an enantioselective thiourea-catalysed Pictet-Spengler reaction, providing the tetrahydro-β-carboline ring and

DNA-Accelerated Copper Catalysis of Friedel-Crafts Conjugate Addition/Enantioselective Protonation Reactions in Water

NARCIS (Netherlands)

García-Fernández, Almudena; Megens, Rik P.; Villarino, Lara; Roelfes, Gerard

2016-01-01

DNA-induced rate acceleration has been identified as one of the key elements for the success of the DNA-based catalysis concept. Here we report on a novel DNA-based catalytic Friedel-Crafts conjugate addition/enantioselective protonation reaction in water, which represents the first example of a

Application of 7-azaisatins in enantioselective Morita–Baylis–Hillman reaction

Directory of Open Access Journals (Sweden)

Qing He

2016-02-01

Full Text Available 7-Azaisatin and 7-azaoxindole skeletons are valuable building blocks in diverse biologically active substances. Here 7-azaisatins turned out to be more efficient electrophiles than the analogous isatins in the enantioselective Morita–Baylis–Hillman (MBH reactions with maleimides using a bifunctional tertiary amine, β-isocupreidine (β-ICD, as the catalyst. This route allows a convenient approach to access multifunctional 3-hydroxy-7-aza-2-oxindoles with high enantiopurity (up to 94% ee. Other types of activated alkenes, such as acrylates and acrolein, could also be efficiently utilized.

An ylide transformation of rhodium(I) carbene: enantioselective three-component reaction through trapping of rhodium(I)-associated ammonium ylides by β-nitroacrylates.

Science.gov (United States)

Ma, Xiaochu; Jiang, Jun; Lv, Siying; Yao, Wenfeng; Yang, Yang; Liu, Shunying; Xia, Fei; Hu, Wenhao

2014-11-24

The chiral Rh(I)-diene-catalyzed asymmetric three-component reaction of aryldiazoacetates, aromatic amines, and β-nitroacrylates was achieved to obtain γ-nitro-α-amino-succinates in good yields and with high diastereo- and enantioselectivity. This reaction is proposed to proceed through the enantioselective trapping of Rh(I)-associated ammonium ylides by nitroacrylates. This new transformation represents the first example of Rh(I)-carbene-induced ylide transformation. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Phosphite radicals and their reactions. Examples of redox, substitution, and addition reactions

International Nuclear Information System (INIS)

Schaefer, K.; Asmus, K.D.

1980-01-01

Phosphite radicals HPO 3 - and PO 3 2 -, which exist in an acid-base equilibrium with pK = 5.75, are shown to take part in various types of reactions. In the absence of scavengers, they disappear mainly by second-order disproportionation and combination; a first-order contribution to the decay is also indicated. HPO 3 - and PO 3 2 - are good reductants toward electron acceptors such as tetranitromethane. In this reaction phosphate and C(NO 2 ) 3 - are formed. Phosphite radicals can, however, also act as good oxidants, e.g., toward thiols and thiolate ions. These reactions lead to the formation of RS. radicals which were identified either directly, as in the case of penicillamine, through the optical absorption of PenS. or more indirectly through equilibration of RS. with RS- to the optically absorbing RSSR-. disulfide radical anion. A homolytic substitution reaction (S/sub H/2) occurs in the reaction of the phosphite radicals with aliphatic disulfides, yielding RS. radicals and phosphate thioester RSPO 3 2 -. Lipoic acid, as an example of a cyclic disulfide, is reduced to the corresponding RSSR-. radical anion and also undergoes the S/sub H/2 reaction with about equal probability. An addition reaction is observed between phosphite radicals and molecular oxygen. The resulting peroxo phosphate radicals establish an acid-base equilibrium HPO 5 - . reversible PO 5 2- . + H+ with a pK = 3.4. Absolute rate constants were determined for all reactions discussed

Hydroxyl-radical-induced oxidation of cyclic dipeptides: Reactions of free peptide radicals and their peroxyl radicals

International Nuclear Information System (INIS)

Mieden, O.J.

1989-01-01

In the course of this study investigations were carried out into the reactions of hydroxyl radicals and hydrogen atoms with cyclic dipeptides as well as the subsequent reactions of peptide radicals and their peroxyl radicals in aqueous solution. The radiolysis products formed in the absence and presence of oxygen or transient metal complexes were characterized and determined on a quantitative basis. The linking of information from product analyses to the kinetic data for transient species obtained by time-resolving UV/VIS and conductivity measurements (pulse radiolysis) as well as computer-assisted simulations of individual events during the reaction permitted an evaluation of the mechanisms underlying the various processes and an identification of interim products with short life-times, which did or did not belong to the group of radicals. Through the characterization of key reactions of radicals and peroxyl radicals of this substance class a major advance has been made towards a better understanding of the role of radicals in the peptide compound and the mechanisms involved in indirect radiation effects on long-chain peptides and proteins. (orig.) [de

Hot wire radicals and reactions

International Nuclear Information System (INIS)

Zheng Wengang; Gallagher, Alan

2006-01-01

Threshold ionization mass spectroscopy is used to measure radical (and stable gas) densities at the substrate of a tungsten hot wire (HW) reactor. We report measurements of the silane reaction probability on the HW and the probability of Si and H release from the HW. We describe a model for the atomic H release, based on the H 2 dissociation model. We note major variations in silicon-release, with dependence on prior silane exposure. Measured radical densities versus silane pressure yield silicon-silane and H-silane reaction rate coefficients, and the dominant radical fluxes to the substrate

Reaction between protein radicals and other biomolecules

DEFF Research Database (Denmark)

Østdal, H.; Davies, M.J.; Andersen, Henrik Jørgen

2002-01-01

The present study investigates the reactivity of bovine serum albumin (BSA) radicals towards different biomolecules (urate, linoleic acid, and a polypeptide, poly(Glu-Ala-Tyr)). The BSA radical was formed at room temperature through a direct protein-to-protein radical transfer from H(2)O(2....... Subsequent analysis showed a decrease in the concentration of urate upon reaction with the BSA radical, while the BSA radical in the presence of poly(Glu-Ala-Tyr) resulted in increased formation of the characteristic protein oxidation product, dityrosine. Reaction between the BSA radical and a linoleic acid...

Spin-Center Shift-Enabled Direct Enantioselective α-Benzylation of Aldehydes with Alcohols.

Science.gov (United States)

Nacsa, Eric D; MacMillan, David W C

2018-03-07

Nature routinely engages alcohols as leaving groups, as DNA biosynthesis relies on the removal of water from ribonucleoside diphosphates by a radical-mediated "spin-center shift" (SCS) mechanism. Alcohols, however, remain underused as alkylating agents in synthetic chemistry due to their low reactivity in two-electron pathways. We report herein an enantioselective α-benzylation of aldehydes using alcohols as alkylating agents based on the mechanistic principle of spin-center shift. This strategy harnesses the dual activation modes of photoredox and organocatalysis, engaging the alcohol by SCS and capturing the resulting benzylic radical with a catalytically generated enamine. Mechanistic studies provide evidence for SCS as a key elementary step, identify the origins of competing reactions, and enable improvements in chemoselectivity by rational photocatalyst design.

The Catalytic Enantioselective Total Synthesis of (+)‐Liphagal

DEFF Research Database (Denmark)

Day, Joshua J.; McFadden, Ryan M.; Virgil, Scott C.

2011-01-01

Ring a ding: The first catalytic enantioselective total synthesis of the meroterpenoid natural product (+)-liphagal is disclosed. The approach showcases a variety of technology including enantioselective enolate alkylation, a photochemical alkyne-alkene [2+2] reaction, microwaveassisted metal...

Reactions of carbonate radical with cobalt(II) aminopolycarboxylates

International Nuclear Information System (INIS)

Mandal, P.C.; Bardhan, D.K.; Bhattacharyya, S.N.

1992-01-01

Reactions of carbonate (CO 3 - radical) and bicarbonate (HCO 3 radical) radicals generated by photolysis of a carbonate or bicarbonate solution at pH 11.2 and 8.5, respectively, with Co(II) complexes of iminodiacetic acid (IDA) and ethylenediaminetetraacetic acid (EDTA) have been studied. The rate constants for the reactions were in the order of 10 6 -10 7 dm 3 mol -1 s -1 . From the time-resolved spectroscopy of the products formed after reaction of CO 3 - radical or HCO 3 radical, it is observed that CO 3 - radical or HCO 3 radical oxidize the metal center to its higher oxidation state. (author) 26 refs.; 2 figs.; 1 tab

Rhodium(II)-catalyzed enantioselective synthesis of troponoids.

Science.gov (United States)

Murarka, Sandip; Jia, Zhi-Jun; Merten, Christian; Daniliuc, Constantin-G; Antonchick, Andrey P; Waldmann, Herbert

2015-06-22

We report a rhodium(II)-catalyzed highly enantioselective 1,3-dipolar cycloaddition reaction between the carbonyl moiety of tropone and carbonyl ylides to afford troponoids in good to high yields with excellent enantioselectivity. We demonstrate that α-diazoketone-derived carbonyl ylides, in contrast to carbonyl ylides derived from diazodiketoesters, undergo [6+3] cycloaddition reactions with tropone to yield the corresponding bridged heterocycles with excellent stereoselectivity. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Unimolecular reaction dynamics of free radicals

International Nuclear Information System (INIS)

Terry A. Miller

2006-01-01

Free radical reactions are of crucial importance in combustion and in atmospheric chemistry. Reliable theoretical models for predicting the rates and products of these reactions are required for modeling combustion and atmospheric chemistry systems. Unimolecular reactions frequently play a crucial role in determining final products. The dissociations of vinyl, CH2= CH, and methoxy, CH3O, have low barriers, about 13,000 cm-1 and 8,000 cm-1, respectively. Since barriers of this magnitude are typical of free radicals these molecules should serve as benchmarks for this important class of reactions. To achieve this goal, a detailed understanding of the vinyl and methoxy radicals is required. Results for dissociation dynamics of vinyl and selectively deuterated vinyl radical are reported. Significantly, H-atom scrambling is shown not to occur in this reaction. A large number of spectroscopic experiments for CH3O and CHD2O have been performed. Spectra recorded include laser induced fluorescence (LIF), laser excited dispersed fluorescence (LEDF), fluorescence dip infrared (FDIR) and stimulated emission pumping (SEP). Such results are critical for implementing dynamics experiments involving the dissociation of methoxy

Cyclopalladated complexes in enantioselective catalysis

Energy Technology Data Exchange (ETDEWEB)

Dunina, Valeria V; Gorunova, Olga N; Zykov, P A; Kochetkov, Konstantin A

2011-01-31

The results of the use of optically active palladacycles in enantioselective catalysis of [3,3]-sigmatropic rearrangements, aldol condensation, the Michael reaction and cross-coupling are analyzed. Reactions with allylic substrates or reagents and some other transformations are considered.

Rapid Hydrogen Shift Reactions in Acyl Peroxy Radicals

DEFF Research Database (Denmark)

Knap, Hasse Christian; Jørgensen, Solvejg

2017-01-01

-shift with X = 6, 7, 8, or 9) in the hydroperoxy acyl peroxy radicals, this H-shift is a reversible reaction and it scrambles between two peroxides, hydroperoxy acyl peroxy and peroxy peroxoic acid radicals. The forward reaction rate constants of the 1,X-OOH H-shift reactions are estimated to be above 103 s–1...... with transition state theory corrected with Eckart quantum tunnelling correction. The ratio between the forward and reverse reaction rate constant of the 1,X-OOH H-shift reactions is around ∼105. Therefore, the equilibrium is pushed toward the production of peroxy peroxoic acid radicals. These very fast 1,X-OOH H......We have used quantum mechanical chemical calculations (CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/aug-cc-pVTZ) to investigate the hydrogen shift (H-shift) reactions in acyl peroxy and hydroperoxy acyl peroxy radicals. We have focused on the H-shift reactions from a hydroperoxy group (OOH) (1,X-OOH H...

A Convergent Enantioselective Total Synthesis of (-)-Perhydrohistrionicotoxin with an Intramolecular Imino Ene-type Reaction as a Key Step

DEFF Research Database (Denmark)

Tanner, David Ackland; Hagberg, Lars

1998-01-01

A convergent enantioselective total synthesis of the neurotoxic spirocyclic alkaloid (-)-perhydrohistrionicotoxin (2) is described. A Lewis acid-mediated intramolecular imine ene-type reaction was used for the key spirocyclisation step (14 to 3, with 3 being obtained as a single diastereoisomer...

Some reactions of oxidizing radicals with enzymes in aqueous solution

International Nuclear Information System (INIS)

Cundall, R.B.; Bisby, R.H.; Hoe, S.T.; Sims, H.E.; Anderson, R.F.

1979-01-01

A range of oxidizing radicals including some inorganic radical anions and the superoxide radical, can be generated by radiolysis of aqueous solutions. These radicals are more selective in their reactions with amino acids than the hydroxyl radical. Factors controlling the apparent reactivity of radical anions with proteins, such as free radical equilibria and ion-binding, are described. The superoxide radical inactivates papain by reaction with the cysteine residue. This reaction has been studied in solutions subjected to radiations of varying linear energy transfer. (Auth.)

Stereoselective reactions. XXXII. Enantioselective deprotonation of 4-tert-butylcyclohexanone by fluorine-containing chiral lithium amides derived from 1-phenylethylamine and 1-(1-naphthyl)ethylamine.

Science.gov (United States)

Aoki, K; Koga, K

2000-04-01

Enantioselective deprotonation of 4-tert-butylcyclohexanone was examined using 1-phenylethylamine- and 1-(1-naphthyl)ethylamine-derived chiral lithium amides having an alkyl or a fluoroalkyl substituent at the amide nitrogen. The lithium amides having a 2,2,2-trifluoroethyl group on the amide nitrogen are easily accessible in both enantiomeric forms, and were found to induce good enantioselectivity in the present reaction.

Reactions of melatonin with radicals in deoxygenated aqueous solution

International Nuclear Information System (INIS)

Stasica, P.; Ulanski, P.; Rosiak, J.M.

1998-01-01

Reactions of melatonin (N-acetyl-5-methoxytryptamine) with radiolytically generated radicals were studied. Reaction of melatonin with OH radicals is diffusion-controlled (k=1.2 x 10 10 dm 3 mol -1 x s -1 ), the main (but not the only one) intermediate being the indolyl-type radical, while the rate constant for the reaction with hydrated electrons is k=4.3 x 10 8 dm 3 x mol -1 x s -1 . Melatonin is capable of scavenging tert-butanol radicals, while its reactivity towards polymer radicals of poly(acrylic acid) and poly(vinyl pyrrolidone) is very low. (author)

Enantioselective addition of nitrones to activated cyclopropanes.

Science.gov (United States)

Sibi, Mukund P; Ma, Zhihua; Jasperse, Craig P

2005-04-27

In this paper, we demonstrate the first examples of chiral Lewis acid catalysis in the formation of tetrahydro-1,2-oxazines with very high enantioselectivity starting with diactivated cyclopropanes and nitrones (>90% yields and ee). Reactions with racemic substituted cyclopropanes provide approximately 1:1 diastereomeric tetrahydro-1,2-oxazine products with high enantioselectivity. Mechanistic information for the formation of the tetrahydro-1,2-oxazines is also detailed.

Chiral Pyridinium Phosphoramide as a Dual Brønsted Acid Catalyst for Enantioselective Diels-Alder Reaction.

Science.gov (United States)

Nishikawa, Yasuhiro; Nakano, Saki; Tahira, Yuu; Terazawa, Kanako; Yamazaki, Ken; Kitamura, Chitoshi; Hara, Osamu

2016-05-06

Chiral pyridinium phosphoramide 1·HX was designed to be a new class of chiral Brønsted acid catalyst in which both the pyridinium proton and the adjacent imide-like proton activated by the electron-withdrawing pyridinium moiety could work cooperatively as strong dual proton donors. The potential of 1·HX was shown in the enantioselective Diels-Alder reactions of 1-amino dienes with various dienophiles including N-unsubstituted maleimide, which has yet to be successfully used in an asymmetric Diels-Alder reaction.

Flow Giese reaction using cyanoborohydride as a radical mediator

Directory of Open Access Journals (Sweden)

Takahide Fukuyama

2013-09-01

Full Text Available Tin-free Giese reactions, employing primary, secondary, and tertiary alkyl iodides as radical precursors, ethyl acrylate as a radical trap, and sodium cyanoborohydride as a radical mediator, were examined in a continuous flow system. With the use of an automated flow microreactor, flow reaction conditions for the Giese reaction were quickly optimized, and it was found that a reaction temperature of 70 °C in combination with a residence time of 10–15 minutes gave good yields of the desired addition products.

Boosting Chemical Stability, Catalytic Activity, and Enantioselectivity of Metal-Organic Frameworks for Batch and Flow Reactions.

Science.gov (United States)

Chen, Xu; Jiang, Hong; Hou, Bang; Gong, Wei; Liu, Yan; Cui, Yong

2017-09-27

A key challenge in heterogeneous catalysis is the design and synthesis of heterogeneous catalysts featuring high catalytic activity, selectivity, and recyclability. Here we demonstrate that high-performance heterogeneous asymmetric catalysts can be engineered from a metal-organic framework (MOF) platform by using a ligand design strategy. Three porous chiral MOFs with the framework formula [Mn 2 L(H 2 O) 2 ] are prepared from enantiopure phosphono-carboxylate ligands of 1,1'-biphenol that are functionalized with 3,5-bis(trifluoromethyl)-, bismethyl-, and bisfluoro-phenyl substituents at the 3,3'-position. For the first time, we show that not only chemical stability but also catalytic activity and stereoselectivity of the MOFs can be tuned by modifying the ligand structures. Particularly, the MOF incorporated with -CF 3 groups on the pore walls exhibits enhanced tolerance to water, weak acid, and base compared with the MOFs with -F and -Me groups. Under both batch and flow reaction systems, the CF 3 -containing MOF demonstrated excellent reactivity, selectivity, and recyclability, affording high yields and enantioselectivities for alkylations of indoles and pyrrole with a range of ketoesters or nitroalkenes. In contrast, the corresponding homogeneous catalysts gave low enantioselectivity in catalyzing the tested reactions.

The role of achiral pyrazolidinone templates in enantioselective Diels-Alder reactions: scope, limitations, and conformational insights.

Science.gov (United States)

Sibi, Mukund P; Stanley, Levi M; Nie, Xiaoping; Venkatraman, Lakshmanan; Liu, Mei; Jasperse, Craig P

2007-01-17

We have evaluated the role of achiral pyrazolidinone templates in conjunction with chiral Lewis acids in room temperature, enantioselective Diels-Alder cycloadditions. The role of the fluxional N(1) substituent was examined, with the bulky 1-naphthylmethyl group providing enantioselectivities up to 99% ee, while templates with smaller fluxional groups gave lower selectivities. High selectivities were also observed in reactions of 7d with chiral Lewis acids derived from relatively small chiral ligands, suggesting the pyrazolidinone templates are capable of relaying stereochemical information from the ligand to the reaction center. Lewis acids capable of adapting square planar geometries, such as Cu(OTf)2, Cu(ClO4)2, and Pd(ClO4)2, were found to be particularly effective at providing high selectivities. Additionally, substitution at the C-5 position of the pyrazolidinone templates has been shown to be critical for optimal selectivity. Reactions of the optimal pyrazolidinone appended with a number of common dienophiles and various dienes demonstrate the utility of this achiral template. Furthermore, catalytic loadings could be lowered to 2.5 mol % with essentially no loss in selectivity. Pi-Pi interactions were evaluated as a means to explain the unusually high selectivity observed at room temperature. Finally, non-C2-symmetric ligands were employed as a test to determine if chiral relay was operative.

Infrared-thermographic screening of the activity and enantioselectivity of enzymes.

Science.gov (United States)

Reetz, M T; Hermes, M; Becker, M H

2001-05-01

The infrared radiation caused by the heat of reaction of an enantioselective enzyme-catalyzed transformation can be detected by modern photovoltaic infrared (IR)-thermographic cameras equipped with focal-plane array detectors. Specifically, in the lipase-catalyzed enantioselective acylation of racemic 1-phenylethanol, the (R)- and (S)-substrates are allowed to react separately in the wells of microtiter plates, the (R)-alcohol showing hot spots in the IR-thermographic images. Thus, highly enantioselective enzymes can be identified at kinetic resolution.

Enantioselective 1,3-dipolar cycloadditions of diazoacetates with electron-deficient olefins.

Science.gov (United States)

Sibi, Mukund P; Stanley, Levi M; Soeta, Takahiro

2007-04-12

[reaction: see text] A general strategy for highly enantioselective 1,3-dipolar cycloaddition of diazoesters to beta-substituted, alpha-substituted, and alpha,beta-disubstituted alpha,beta-unsaturated pyrazolidinone imides is described. Cycloadditions utilizing less reactive alpha,beta-disubstituted dipolarophiles require elevated reaction temperatures, but still provide the corresponding pyrazolines with excellent enantioselectivities. Finally, an efficient synthesis of (-)-manzacidin A employing this cycloaddition methodology as a key step is illustrated.

Reaction kinetics of resveratrol with thiyl and alkoxyl radicals

International Nuclear Information System (INIS)

Dzeba, I.; Mihaljevic, B.

2011-01-01

Complete text of publication follows. Plant derived resveratrol (trans-3,5,4'-trihydroxystilbene) possesses a broad spectrum of biological activities, one of them are very well known its antioxidative properties. Our work aims to provide kinetic data with regard to the reactivity of resveratrol with uninvestigated short-lived bioradicals, identified as mediators in oxidative lipid degradation processes. Radicals of our interest are alkoxyl radicals, well known propagators of the chain free radical reactions in lipids, and thiyl radicals which protect lipids from their degradation pathway, but at the same time cause the isomerization of the double bonds. In order to investigate these reactions of resveratrol laser flash photolysis was used. On the basis of competitive kinetics the rate constants were determined under pseudo-first order conditions in acetonitrile solutions at room temperature. Thiyl radicals were generated indirectly in solution containing 1-octadecanthiol and photosensitive benzophenone in acetonitrile using the light pulses at 347 nm from ruby laser. Tert-butoxyl radicals were generated directly by peroxide bond cleavage from di-tert-butyl peroxide in acetonitrile by light pulses of Nd:YAG at 355 nm, and ruby at 347 nm. Obtained rate constants for the reactions of resveratrol and radicals generated by laser flash photolysis will be summarized and compared with rare literature data for the rate constants of investigated reactions of resveratrol and other radicals generated by pulse radiolysis.

Fluxional additives: a second generation control in enantioselective catalysis.

Science.gov (United States)

Sibi, Mukund P; Manyem, Shankar; Palencia, Hector

2006-10-25

The concept of "fluxional additives", additives that can adopt enantiomeric conformations depending on the chiral information in the ligand, is demonstrated in enantioselective Diels-Alder and nitrone cycloaddition reactions. The additive design is modular, and diverse structures are accessible in three steps. Chiral Lewis acids from main group and transition metals show enhancements in enantioselectivity in the presence of these additives.

A Tunable and Enantioselective Hetero-Diels-Alder Reaction Provides Access to Distinct Piperidinoyl Spirooxindoles.

Science.gov (United States)

Jayakumar, Samydurai; Louven, Kathrin; Strohmann, Carsten; Kumar, Kamal

2017-12-11

The active complexes of chiral N,N'-dioxide ligands with dysprosium and magnesium salts catalyze the hetero-Diels-Alder reaction between 2-aza-3-silyloxy-butadienes and alkylidene oxindoles to selectively form 3,3'- and 3,4'-piperidinoyl spirooxindoles, respectively, in very high yields and with excellent enantioselectivities. The exo-selective asymmetric cycloaddition successfully regaled the construction of sp 3 -rich and highly substituted natural-product-based spirooxindoles supporting many chiral centers, including contiguous all-carbon quaternary centers. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Insight into the Reaction Mechanism of Graphene Oxide with Oxidative Free Radical

Institute of Scientific and Technical Information of China (English)

ZHOU Xuejiao; XU Liangyou

2017-01-01

Graphene oxide(GO),as an important derivative of graphene,could be considered as a super aromatic molecule decorated with a range of reactive oxygen-containing groups on its surface,which endows graphene high reactivity with other molecules.In our previous work,we demonstrated that GO sheets were cut into small pieces(graphene quantum dots,GQDs) by oxidative free radicals(hydroxyl radical HO or oxygen radical [O]) under UV irradiation.It is notable that reactions involving free radicals are influenced by reaction conditions pronouncedly.However,researches on details about reactions of GO with free radicals have not been reported thus far.In this work,the effects of different factors on the photo-Fenton reaction of GO were studied.It is demonstrated that the reaction rate is closely related to the concentration of free radicals.It is speculated that through the optimization of reaction conditions,the reaction of graphene with free radicals could carry out efficiently for further applications.

Enantioselective cycloadditions with alpha,beta-disubstituted acrylimides.

Science.gov (United States)

Sibi, Mukund P; Ma, Zhihua; Itoh, Kennosuke; Prabagaran, Narayanasamy; Jasperse, Craig P

2005-06-09

[reaction: see text] The use of N-H imide templates provides a solution to the problem of rotamer control in Lewis acid catalyzed reactions of alpha,beta-disubstituted acryloyl imides. Reactions proceed through the s-cis rotamer and with improved reactivity because A(1,3) strain is avoided. Enantioselective nitrone, nitrile oxide, and Diels-Alder cycloadditions demonstrate the principle.

Calcium(ii)-catalyzed enantioselective conjugate additions of amines.

Science.gov (United States)

Uno, Brice E; Dicken, Rachel D; Redfern, Louis R; Stern, Charlotte M; Krzywicki, Greg G; Scheidt, Karl A

2018-02-14

The direct enantioselective chiral calcium(ii)·phosphate complex (Ca[CPA] 2 )-catalyzed conjugate addition of unprotected alkyl amines to maleimides was developed. This mild catalytic system represents a significant advance towards the general convergent asymmetric amination of α,β-unsaturated electrophiles, providing medicinally relevant chiral aminosuccinimide products in high yields and enantioselectivities. Furthermore, the catalyst can be reused directly from a previously chromatographed reaction and still maintain both high yield and selectivity.

Enantioselective Synthesis of α-Mercapto-β-amino Esters via Rh(II)/Chiral Phosphoric Acid-Cocatalyzed Three-Component Reaction of Diazo Compounds, Thiols, and Imines.

Science.gov (United States)

Xiao, Guolan; Ma, Chaoqun; Xing, Dong; Hu, Wenhao

2016-12-02

An enantioselective method for the synthesis of α-mercapto-β-amino esters has been developed via a rhodium(II)/chiral phosphoric acid-cocatalyzed three-component reaction of diazo compounds, thiols, and imines. This transformation is proposed to proceed through enantioselective trapping of the sulfonium ylide intermediate generated in situ from the diazo compound and thiol by the phosphoric acid-activated imine. With this method, a series of α-mercapto-β-amino esters were obtained in good yields with moderate to good stereoselectivities.

Comparing Positively and Negatively Charged Distonic Radical Ions in Phenylperoxyl Forming Reactions.

Science.gov (United States)

Williams, Peggy E; Marshall, David L; Poad, Berwyck L J; Narreddula, Venkateswara R; Kirk, Benjamin B; Trevitt, Adam J; Blanksby, Stephen J

2018-06-04

In the gas phase, arylperoxyl forming reactions play a significant role in low-temperature combustion and atmospheric processing of volatile organic compounds. We have previously demonstrated the application of charge-tagged phenyl radicals to explore the outcomes of these reactions using ion trap mass spectrometry. Here, we present a side-by-side comparison of rates and product distributions from the reaction of positively and negatively charge tagged phenyl radicals with dioxygen. The negatively charged distonic radical ions are found to react with significantly greater efficiency than their positively charged analogues. The product distributions of the anion reactions favor products of phenylperoxyl radical decomposition (e.g., phenoxyl radicals and cyclopentadienone), while the comparable fixed-charge cations yield the stabilized phenylperoxyl radical. Electronic structure calculations rationalize these differences as arising from the influence of the charged moiety on the energetics of rate-determining transition states and reaction intermediates within the phenylperoxyl reaction manifold and predict that this influence could extend to intra-molecular charge-radical separations of up to 14.5 Å. Experimental observations of reactions of the novel 4-(1-carboxylatoadamantyl)phenyl radical anion confirm that the influence of the charge on both rate and product distribution can be modulated by increasing the rigidly imposed separation between charge and radical sites. These findings provide a generalizable framework for predicting the influence of charged groups on polarizable radicals in gas phase distonic radical ions. Graphical Abstract.

Enantioselective copper catalysed intramolecular C-H insertion reactions of α-diazo-β-keto sulfones, α-diazo-β-keto phosphine oxides and 2-diazo-1,3-diketones; the influence of the carbene substituent.

Science.gov (United States)

Shiely, Amy E; Slattery, Catherine N; Ford, Alan; Eccles, Kevin S; Lawrence, Simon E; Maguire, Anita R

2017-03-22

Enantioselectivities in C-H insertion reactions, employing the copper-bis(oxazoline)-NaBARF catalyst system, leading to cyclopentanones are highest with sulfonyl substituents on the carbene carbon, and furthermore, the impact is enhanced by increased steric demand on the sulfonyl substituent (up to 91%ee). Enantioselective intramolecular C-H insertion reactions of α-diazo-β-keto phosphine oxides and 2-diazo-1,3-diketones are reported for the first time.

Transverse flow reactor studies of the dynamics of radical reactions

Energy Technology Data Exchange (ETDEWEB)

Macdonald, R.G. [Argonne National Laboratory, IL (United States)

1993-12-01

Radical reactions are in important in combustion chemistry; however, little state-specific information is available for these reactions. A new apparatus has been constructed to measure the dynamics of radical reactions. The unique feature of this apparatus is a transverse flow reactor in which an atom or radical of known concentration will be produced by pulsed laser photolysis of an appropriate precursor molecule. The time dependence of individual quantum states or products and/or reactants will be followed by rapid infrared laser absorption spectroscopy. The reaction H + O{sub 2} {yields} OH + O will be studied.

The origin of enantioselectivity in the l-threonine-derived phosphine-sulfonamide catalyzed aza-Morita-Baylis-Hillman reaction: Effects of the intramolecular hydrogen bonding

KAUST Repository

Lee, Richmond; Zhong, Fangrui; Zheng, Bin; Meng, Yuezhong; Lu, Yixin; Huang, Kuo-Wei

2013-01-01

in inducing a high degree of stereochemical control in both the enolate addition to imine and the subsequent proton transfer step, affording aza-MBH reactions with excellent enantioselectivity. © 2013 The Royal Society of Chemistry.

Enantioselective rhodium enolate protonations. A new methodology for the synthesis of beta2-amino acids.

Science.gov (United States)

Sibi, Mukund P; Tatamidani, Hiroto; Patil, Kalyani

2005-06-23

[reaction: see text] Rhodium-catalyzed conjugate addition of an aryl boronic acid to alpha-methylamino acrylates followed by enantioselective protonation of the oxa-pi-allylrhodium intermediate provides access to aryl-substituted beta(2)-amino acids. The impact of the different variables of the reaction on the levels of enantioselectivity has been assessed.

The Brønsted Acid-Catalyzed, Enantioselective Aza-Diels-Alder Reaction for the Direct Synthesis of Chiral Piperidones.

Science.gov (United States)

Weilbeer, Claudia; Sickert, Marcel; Naumov, Sergei; Schneider, Christoph

2017-01-12

We disclose herein the first enantioselective aza-Diels-Alder reaction of β-alkyl-substituted vinylketene silyl-O,O-acetals and imines furnishing a broad range of optically highly enriched 4-alkyl-substituted 2-piperidones. As a catalyst for this one-pot reaction we employed a chiral phosphoric acid which effects a vinylogous Mannich reaction directly followed by ring-closure to the lactam. Subsequent fully diastereoselective transformations including hydrogenation, enolate alkylation, and lactam alkylation/reduction processes converted the cycloadducts into various highly substituted piperidines of great utility for the synthesis of natural products and medicinally active compounds. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Configuration of a pulse radiolysis system for the study of gas-phase reactions and kinetic investigations of the reactions of hydroxyl radicals with methyl and ethyl radicals

International Nuclear Information System (INIS)

Fagerstroem, K.

1993-01-01

The work that is presented in this thesis deals with the assembling and testing of a pulse radiolysis system for kinetic studies of gas-phase reactions as well as with the kinetics of the gas-phase reactions of hydroxyl radicals with methyl and ethyl radicals. These radicals are very important as these are formed at an early stage in hydrocarbon combustion processes. The two studied reactions are key reactions in those processes. (6 refs., 4 figs., 2 tabs.)

Ring-Expansion/Contraction Radical Crossover Reactions of Cyclic Alkoxyamines: A Mechanism for Ring Expansion-Controlled Radical Polymerization

Directory of Open Access Journals (Sweden)

Atsushi Narumi

2018-06-01

Full Text Available Macrocyclic polymers present an important class of macromolecules, displaying the reduced radius of gyration or impossibility to entangle. A rare approach for their synthesis is the ring expansion-controlled radical “vinyl” polymerization, starting from a cyclic alkoxyamine. We here describe ring-expansion radical crossover reactions of cyclic alkoxyamines which run in parallel to chain-propagation reactions in the polymerization system. The radical crossover reactions extensively occurred at 105–125 °C, eventually producing high molecular weight polymers with multiple inherent dynamic covalent bonds (NOC bonds. A subsequent ring-contraction radical crossover reaction and the second ring-expansion radical crossover reaction are also described. The major products for the respective three stages were shown to possess cyclic morphologies by the molecular weight profiles and the residual ratios for the NOC bonds (φ in %. In particular, the high φ values ranging from ca. 80% to 98% were achieved for this cyclic alkoxyamine system. This result verifies the high availability of this system as a tool demonstrating the ring-expansion “vinyl” polymerization that allows them to produce macrocyclic polymers via a one-step vinyl polymerization.

One-Pot Synthesis of Novel Chiral β-Amino Acid Derivatives by Enantioselective Mannich Reactions Catalyzed by Squaramide Cinchona Alkaloids

Directory of Open Access Journals (Sweden)

Kankan Zhang

2013-05-01

Full Text Available An efficient one-pot synthesis of novel β-amino acid derivatives containing a thiadiazole moiety was developed using a chiral squaramide cinchona alkaloid as organocatalyst. The reactions afforded chiral β-amino acid derivatives in moderate yields and with moderate to excellent enantioselectivities. The present study demonstrated for the first time the use of a Mannich reaction catalyzed by a chiral bifunctional organocatalyst for the one-pot synthesis of novel β-amino acid derivatives bearing a 1,3,4-thiadiazole moiety on nitrogen.

Reactions of linoleic acid peroxyl radicals with phenolic antioxidants: a pulse radiolysis study

International Nuclear Information System (INIS)

Erben-Russ, Michael; Bors, Wolf; Saran, Manfred

1987-01-01

Linoleic acid peroxyl radicals (LOO) can be viewed as model intermediates occurring during lipid peroxidation processes. Formation and reactions of these species were investigated in aqueous alkaline solution using pulse radiolysis combined with kinetic spectroscopy. Irradiation of linoleic acid in N 2 O/O 2 -saturated solutions leads to a mixture of peroxyl radical isomers; reaction of 13-hydroperoxylinoleic acid (13-LOOH) with azide radicals in N 2 O-saturated solution produces 13-LOO radicals specifically. These peroxyl radicals cannot be observed directly, but their reactions with kaempferol and quercetin, acting as radical-scavenging antioxidants, produced strongly absorbing aroxyl radicals (ArO). The same aroxyl radicals were generated by OH and N 3 with rate constants exceeding 10 9 dm 3 mol -1 s -1 . Applying a reaction scheme that includes competing generation and decay reactions of both LOO and ArO radicals, individual rate constants were derived for LOO reactions with the phenols (> 10 7 dm 3 mol -1 s-? 1 ), with aroxyl radicals to form covalent adducts (> 10 8 dm 3 mol -1 s -1 ), as well as for their bimilecular decay (3.0 x 10 8 dm 3 mol -1 s -1 ). These results demonstrate high reactivity of fatty acid peroxyl radicals and flavone antioxidants in aqueous solution. (author)

Peroxyl radical reactions with carotenoids in microemulsions: Influence of microemulsion composition and the nature of peroxyl radical precursor.

Science.gov (United States)

El-Agamey, Ali; McGarvey, David J

2016-01-01

The reactions of acetylperoxyl radicals with different carotenoids (7,7'-dihydro-β-carotene and ζ-carotene) in SDS and CTAC microemulsions of different compositions were investigated using laser flash photolysis (LFP) coupled with kinetic absorption spectroscopy. The primary objective of this study was to explore the influence of microemulsion composition and the type of surfactant used on the yields and kinetics of various transients formed from the reaction of acetylperoxyl radicals with carotenoids. Also, the influence of the site (hydrocarbon phases or aqueous phase) of generation of the peroxyl radical precursor was examined by using 4-acetyl-4-phenylpiperidine hydrochloride (APPHCl) and 1,1-diphenylacetone (11DPA) as water-soluble and lipid-soluble peroxyl radical precursors, respectively. LFP of peroxyl radical precursors with 7,7'-dihydro-β-carotene (77DH) in different microemulsions gives rise to the formation of three distinct transients namely addition radical (λmax=460 nm), near infrared transient1 (NIR, λmax=700 nm) and 7,7'-dihydro-β-carotene radical cation (77DH(•+), λmax=770 nm). In addition, for ζ-carotene (ZETA) two transients (near infrared transient1 (NIR1, λmax=660 nm) and ζ-carotene radical cation (ZETA(•+), λmax=730-740 nm)) are generated following LFP of peroxyl radical precursors in the presence of ζ-carotene (ZETA) in different microemulsions. The results show that the composition of the microemulsion strongly influences the observed yield and kinetics of the transients formed from the reactions of peroxyl radicals (acetylperoxyl radicals) with carotenoids (77DH and ZETA). Also, the type of surfactant used in the microemulsions influences the yield of the transients formed. The dependence of the transient yields and kinetics on microemulsion composition (or the type of surfactant used in the microemulsion) can be attributed to the change of the polarity of the microenvironment of the carotenoid. Furthermore, the nature of

Reactions of linoleic acid peroxyl radicals with phenolic antioxidants: a pulse radiolysis study

Energy Technology Data Exchange (ETDEWEB)

Erben-Russ, M.; Bors, W.; Saran, M.

1987-09-01

Linoleic acid peroxyl radicals (LOO) can be viewed as model intermediates occurring during lipid peroxidation processes. Formation and reactions of these species were investigated in aqueous alkaline solution using pulse radiolysis combined with kinetic spectroscopy. Irradiation of linoleic acid in N/sub 2/O/O/sub 2/-saturated solutions leads to a mixture of peroxyl radical isomers; reaction of 13-hydroperoxylinoleic acid (13-LOOH) with azide radicals in N/sub 2/O-saturated solution produces 13-LOO radicals specifically. These peroxyl radicals cannot be observed directly, but their reactions with kaempferol and quercetin, acting as radical-scavenging antioxidants, produced strongly absorbing aroxyl radicals (ArO). The same aroxyl radicals were generated by OH and N/sub 3/ with rate constants exceeding 10/sup 9/ dm/sup 3/ mol/sup -1/ s/sup -1/. Applying a reaction scheme that includes competing generation and decay reactions of both LOO and ArO radicals, individual rate constants were derived for LOO reactions with the phenols (> 10/sup 7/ dm/sup 3/ mol/sup -1/ s-./sup 1/), with aroxyl radicals to form covalent adducts (> 10/sup 8/ dm/sup 3/ mol/sup -1/ s/sup -1/), as well as for their bimilecular decay (3.0 x 10/sup 8/ dm/sup 3/ mol/sup -1/ s/sup -1/). These results demonstrate high reactivity of fatty acid peroxyl radicals and flavone antioxidants in aqueous solution.

Reactions of Hydroxyalkyl Radicals with Cysteinyl Peptides in a NanoESI Plume

Science.gov (United States)

Stinson, Craig A.; Xia, Yu

2014-07-01

In biological systems, carbon-centered small molecule radicals are primarily formed via external radiation or internal radical reactions. These radical species can react with a variety of biomolecules, most notably nucleic acids, the consequence of which has possible links to gene mutation and cancer. Sulfur-containing peptides and proteins are reactive toward a variety of radical species and many of them behave as radical scavengers. In this study, the reactions between alkyl alcohol carbon-centered radicals (e.g., •CH2OH for methanol) and cysteinyl peptides within a nanoelectrospray ionization (nanoESI) plume were explored. The reaction system involved ultraviolet (UV) irradiation of a nanoESI plume using a low pressure mercury lamp consisting of 185 and 254 nm emission bands. The alkyl alcohol was added as solvent into the nanoESI solution and served as the precursor of hydroxyalkyl radicals upon UV irradiation. The hydroxyalkyl radicals subsequently reacted with cysteinyl peptides either containing a disulfide linkage or free thiol, which led to the formation of peptide- S-hydroxyalkyl product. This radical reaction coupled with subsequent MS/MS was shown to have analytical potential by cleaving intrachain disulfide linked peptides prior to CID to enhance sequence information. Tandem mass spectrometry via collision-induced dissociation (CID), stable isotope labeling, and accurate mass measurement were employed to verify the identities of the reaction products.

Formation and reactions of radical cations of substituted benzenes in aqueous media

International Nuclear Information System (INIS)

Holcman, J.

1977-08-01

Radical cations of anisole, methylated benzenes, ethylbenzene, isopropylbenzene, tert-butylbenzene and N,N-dimethylaniline were studied in aqueous media by pulse radiolytic technique. Absorption spectra and reaction kinetics of the radical cations were recorded. The radical cations are formed from the corresponding OH adducts by the elimination of OH - , either by a simple dissociation or by an acid catalyzed reaction. The rate constants of the formation of the radical cations and their reactions with water, OH - and Fe 2+ , or the reaction of a proton loss, were measured. The rate constants for the reaction with water and OH - , together with the rate constants for the dissociation of the OH adducts, are correlated with the ionization potential of the parent compound. These correlations offer a possibility of predicting the acid-base properties of radical cations of substituted benzenes, or the estimation of their ionization potential. (author)

Time-resolved FTIR emission studies of laser photofragmentation and radical reactions

Energy Technology Data Exchange (ETDEWEB)

Leone, S.R. [Univ. of Colorado, Boulder (United States)

1993-12-01

Recent studies have focused specifically on collision processes, such as single collision energy transfer, reaction dynamics, and radical reactions. The authors employ novel FTIR techniques in the study of single collision energy transfer processes using translationally fast H atom, as well as radical-radical reactions, e.g. CH{sub 3} + O, CF{sub 3} + H(D), and Cl + C{sub 2}H{sub 5}. The fast atoms permit unique high energy regions of certain transition states of combustion species to be probed for the first time.

Engaging unactivated alkyl, alkenyl and aryl iodides in visible-light-mediated free radical reactions

Science.gov (United States)

Nguyen, John D.; D'Amato, Erica M.; Narayanam, Jagan M. R.; Stephenson, Corey R. J.

2012-10-01

Radical reactions are a powerful class of chemical transformations. However, the formation of radical species to initiate these reactions has often required the use of stoichiometric amounts of toxic reagents, such as tributyltin hydride. Recently, the use of visible-light-mediated photoredox catalysis to generate radical species has become popular, but the scope of these radical precursors has been limited. Here, we describe the identification of reaction conditions under which photocatalysts such as fac-Ir(ppy)3 can be utilized to form radicals from unactivated alkyl, alkenyl and aryl iodides. The generated radicals undergo reduction via hydrogen atom abstraction or reductive cyclization. The reaction protocol utilizes only inexpensive reagents, occurs under mild reaction conditions, and shows exceptional functional group tolerance. Reaction efficiency is maintained upon scale-up and decreased catalyst loading, and the reaction time can be significantly shortened when the reaction is performed in a flow reactor.

Crossed-beam studies of the dynamics of radical reactions

Energy Technology Data Exchange (ETDEWEB)

Liu, K. [Argonne National Laboratory, IL (United States)

1993-12-01

The objective of this program is to characterize the detailed dynamics of elementary radical reactions and to provide a better understanding of radical reactivity in general. The radical beam is typically generated by a laser photolysis method. After colliding with the reacting molecule in a crossed-beam apparatus, the reaction product state distribution is interrogated by laser spectroscopic techniques. Several radicals of combustion significance, such as O, CH, OH, CN and NCO have been successfully generated and their collisional behavior at the state-to-state integral cross section level of detail has been studied in this manner. During the past year, the detection system has been converted from LIF to REMPI schemes, and the emphasis of this program shifted to investigate the product angular distributions. Both inelastic and reactive processes have been studied.

Synthesis of l-threitol-based crown ethers and their application as enantioselective phase transfer catalyst in Michael additions.

Science.gov (United States)

Rapi, Zsolt; Nemcsok, Tamás; Pálvölgyi, Ádám; Keglevich, György; Grün, Alajos; Bakó, Péter

2017-06-01

A few new l-threitol-based lariat ethers incorporating a monoaza-15-crown-5 unit were synthesized starting from diethyl l-tartrate. These macrocycles were used as phase transfer catalysts in asymmetric Michael addition reactions under mild conditions to afford the adducts in a few cases in good to excellent enantioselectivities. The addition of 2-nitropropane to trans-chalcone, and the reaction of diethyl acetamidomalonate with β-nitrostyrene resulted in the chiral Michael adducts in good enantioselectivities (90% and 95%, respectively). The substituents of chalcone had a significant impact on the yield and enantioselectivity in the reaction of diethyl acetoxymalonate. The highest enantiomeric excess (ee) values (99% ee) were measured in the case of 4-chloro- and 4-methoxychalcone. The phase transfer catalyzed cyclopropanation reaction of chalcone and benzylidene-malononitriles using diethyl bromomalonate as the nucleophile (MIRC reaction) was also developed. The corresponding chiral cyclopropane diesters were obtained in moderate to good (up to 99%) enantioselectivities in the presence of the threitol-based crown ethers. © 2017 Wiley Periodicals, Inc.

Reactions of carbon radicals generated by 1,5-transposition of reactive centers

Directory of Open Access Journals (Sweden)

ZIVORAD CEKOVIC

2005-03-01

Full Text Available Radical intermediates can undergo specific reactions, such as intramolecular rearrangements, i.e., the transpositions of radical centers, which are not known in classical ionic organic reactions. 1,5-Transposition of a radical center to a non-activated carbon atom are of great synthetic importance. It can be successfully applied for the introduction of different functional groups (oxygen, nitrogen, sulfur, halogens onto a carbon atom remote from the present functional group. In addition to functionalization of a remote non-activated carbon atom, the formation of new C-C bonds on the d-carbon atom have also been achieved. 1,5-Transposition of the radical centers takes place from alkoxyl, aminyl and carbon radicals to a remote carbon atom. Relocation of the radical centers preferentially involves 1,5-transfer of a hydrogen atom, although migrations of some other groups are known. The reactions of the carbon radical generated by 1,5-relocation of the radical center are presented and their synthetic applications are reviewed.

Catalytic Enantioselective Alkylation of β-Keto Esters with Xanthydrol in the Presence of Chiral Palladium Complex

Energy Technology Data Exchange (ETDEWEB)

Kim, Kyu Yeon; Kim, Dae Young [Soonchunhyang Univ., Asan (Korea, Republic of)

2016-01-15

Our research interest has been directed toward the development of synthetic methods for the enantioselective construction of stereogenic carbon centers. Recently, we explored the catalytic enantioselective functionalization of active methines in the presence of chiral palladium(II) complexes. In conclusion, we have accomplished the efficient catalytic enantioselective alkylation of β-keto esters 1 with xanthydrol 2 with high yields and excellent enantioselectivity (up to 98% ee). It should be noted that this alkyaltion reaction proceeds well using air- and moisture-stable chiral palladium com- plexes with low loading (1 mol%)

The kinetics of free radical metathetical and addition reactions in silane solutions

International Nuclear Information System (INIS)

Aloni, R.

1976-12-01

In this work radiolytic technique was employed for the initiation of free radical chainreactions in silane solution. The kinetic analysis of the chain mechanism in various solutions enabled the determination of the Arrhenius parameters for metathesis, addition and unimolecular decomposition reactions which make up the chainpropagation sequence in the systems studied. The following radical reactions were investigated: chlorine atom abstraction from chloromethanes by SiCl 3 and Et 3 Si radicals, and chlorine atom abstraction from chloroethanes by Et 3 Si radicals; unimolecular decomposition reactions and hydrogen atom abstraction, *from the solvent, of chloroethyl radicals in triethylsilane solutions; addition and abstraction reactions of Et 3 Si radicals with chloroolefins. Arrhenius parameters were determined for abstraction of chlorine atom from CH 3 Cl, CH 2 Cl 2 , CHCl 3 and CCl 4 , by SiCl 3 radicals and from CCl 4 , CHCl 3 , CH 2 Cl 2 , CCl 3 CN, C 2 Cl 5 H, sym-C 2 Cl 4 H 2 , asym-C 2 Cl 4 H 2 , 1.1.1-C 2 Cl 3 H 3 , 1.1.1-C 2 Dl 3 F 3 and 1.1-C 2 Cl 2 H 4 by Et 3 Si radicals. (author)

Development of linear free energy relationships for aqueous phase radical-involved chemical reactions.

Science.gov (United States)

Minakata, Daisuke; Mezyk, Stephen P; Jones, Jace W; Daws, Brittany R; Crittenden, John C

2014-12-02

Aqueous phase advanced oxidation processes (AOPs) produce hydroxyl radicals (HO•) which can completely oxidize electron rich organic compounds. The proper design and operation of AOPs require that we predict the formation and fate of the byproducts and their associated toxicity. Accordingly, there is a need to develop a first-principles kinetic model that can predict the dominant reaction pathways that potentially produce toxic byproducts. We have published some of our efforts on predicting the elementary reaction pathways and the HO• rate constants. Here we develop linear free energy relationships (LFERs) that predict the rate constants for aqueous phase radical reactions. The LFERs relate experimentally obtained kinetic rate constants to quantum mechanically calculated aqueous phase free energies of activation. The LFERs have been applied to 101 reactions, including (1) HO• addition to 15 aromatic compounds; (2) addition of molecular oxygen to 65 carbon-centered aliphatic and cyclohexadienyl radicals; (3) disproportionation of 10 peroxyl radicals, and (4) unimolecular decay of nine peroxyl radicals. The LFERs correlations predict the rate constants within a factor of 2 from the experimental values for HO• reactions and molecular oxygen addition, and a factor of 5 for peroxyl radical reactions. The LFERs and the elementary reaction pathways will enable us to predict the formation and initial fate of the byproducts in AOPs. Furthermore, our methodology can be applied to other environmental processes in which aqueous phase radical-involved reactions occur.

Investigating radical cation chain processes in the electrocatalytic Diels-Alder reaction.

Science.gov (United States)

Imada, Yasushi; Okada, Yohei; Chiba, Kazuhiro

2018-01-01

Single electron transfer (SET)-triggered radical ion-based reactions have proven to be powerful options in synthetic organic chemistry. Although unique chain processes have been proposed in various photo- and electrochemical radical ion-based transformations, the turnover number, also referred to as catalytic efficiency, remains unclear in most cases. Herein, we disclose our investigations of radical cation chain processes in the electrocatalytic Diels-Alder reaction, leading to a scalable synthesis. A gram-scale synthesis was achieved with high current efficiency of up to 8000%. The reaction monitoring profiles showed sigmoidal curves with induction periods, suggesting the involvement of intermediate(s) in the rate determining step.

[Development of boomerang-type intramolecular cascade reactions and application to natural product synthesis].

Science.gov (United States)

Takasu, K

2001-12-01

Intramolecular cascade reaction has received much attention as a powerful methodology to construct a polycyclic framework in organic synthesis. We have been developing "boomerang-type cascade reaction" to construct a variety of polycyclic skeletons efficiently. In the above reactions, a nucleophilic function of substrates changes the character into an electrophile after the initial reaction, and the electrophilic group acts as a nucleophile in the second reaction. That is, the reaction center stepwise moves from one functional group back to the same one via other functional groups. The stream of the electron concerning the cascade reaction is like a locus of boomerang. We show here three different boomerang-type reactions via ionic species or free radicals. 1) Diastereoselective Michael-aldol reaction based on the chiral auxiliary method and enantioselective Michael-aldol reaction by the use of external chiral sources. 2) Short and efficient total syntheses of longifolane sesquiterpenes utilizing intramolecular double Michael addition as a key step. 3) Development of boomerang-type radical cascade reaction of halopolyenes to construct terpenoid skeletons and its regioselectivity.

Comparative study of radical oxidation of DNA and its nucleosides by hydroxyl radicals and ferryl ions generated by the Fenton reaction

International Nuclear Information System (INIS)

Mouret, J.F.; Berger, M.; Anselmino, C.; Polverelli, M.; Cadet, J.

1991-01-01

A comparative study of the reaction of hydroxyl radicals and Fenton type oxidative species with DNA and 2'-deoxyribonucleosides was investigated. This study was based on the characterization of the diamagnetic products resulting from the chemical transformation of the transient radicals. Emphasis was placed on the radical oxidative reactions of the purine nucleosides. It is interesting to note that oxidative purine radicals can be reduced by reagents such as ascorbic acid or N,N,N',N'-tetramethyl-1, 4-p-phenylenediamine. The observed differences in the nature of the decomposition products resulting from the Fenton reaction are not consistent with the nature of the oxidative species (hydroxyl radicals or ferryl ions) involved, but due to the presence of ferrous sulfate [fr

Enantioselective Rhodium Enolate Protonations. A New Methodology for the Synthesis of β2-Amino Acids

Science.gov (United States)

Sibi, Mukund P.; Tatamidani, Hiroto; Patil, Kalyani

2008-01-01

Rhodium catalyzed conjugate addition of an aryl boronic acid to α-methylamino acrylates followed by enantioselective protonation of the oxa-π-allylrhodium intermediate provides access to aryl substituted β2-amino acids. The impact of the different variables of the reaction on the levels of enantioselectivity has been assessed. PMID:15957893

Scavenging of free-radical metabolites of aniline xenobiotics and drugs by amino acid derivatives: toxicological implications of radical-transfer reactions.

Science.gov (United States)

Michail, Karim; Baghdasarian, Argishti; Narwaley, Malyaj; Aljuhani, Naif; Siraki, Arno G

2013-12-16

We investigated a novel scavenging mechanism of arylamine free radicals by poly- and monoaminocarboxylates. Free radicals of arylamine xenobiotics and drugs did not react with oxygen in peroxidase-catalyzed reactions; however, they showed marked oxygen uptake in the presence of an aminocarboxylate. These free-radical intermediates were identified using the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and electron paramagnetic resonance (EPR) spectrometry. Diethylenetriaminepentaacetic acid (DTPA), a polyaminocarboxylate, caused a concentration-dependent attenuation of N-centered radicals produced by the peroxidative metabolism of arylamines with the subsequent formation of secondary aliphatic carbon-centered radicals stemming from the cosubstrate molecule. Analogously, N,N-dimethylglycine (DMG) and N-methyliminodiacetate (MIDA), but not iminodiacetic acid (IDA), demonstrated a similar scavenging effect of arylamine-derived free radicals in a horseradish peroxidase/H2O2 system. Using human promyelocytic leukemia (HL-60) cell lysate as a model of human neutrophils, DTPA, MIDA, and DMG readily reduced anilinium cation radicals derived from the arylamines and gave rise to the corresponding carbon radicals. The rate of peroxidase-triggered polymerization of aniline was studied as a measure of nitrogen-radical scavenging. Although, IDA had no effect on the rate of aniline polymerization, this was almost nullified in the presence of DTPA and MIDA at half of the molar concentration of the aniline substrate, whereas a 20 molar excess of DMPO caused only a partial inhibition. Furthermore, the yield of formaldehyde, a specific reaction endproduct of the oxidation of aminocarboxylates by aniline free-radical metabolites, was quantitatively determined. Azobenzene, a specific reaction product of peroxidase-catalyzed free-radical dimerization of aniline, was fully abrogated in the presence of DTPA, as confirmed by GC/MS. Under aerobic conditions, a radical-transfer reaction

Novel Profluorescent Nitroxides for Monitoring Alkyl Radical Reactions During Radiation Degradation

International Nuclear Information System (INIS)

George, G.

2006-01-01

Hindered amine stabilizers (HAS) are effective at retarding the photo-oxidative and high energy radiation degradation of PP and in certain circumstances, also thermo-oxidative degradation. The effectiveness of HAS as retarders of oxidation relies on the oxidation of the N-C bond by polymer hydroperoxide, ROOH, to form the nitroxyl group -NO which is the scavenger of polymer alkyl radicals, R. This reaction, which produces the alkoxy amine: -NO-R, must be competitive with the reaction of R with oxygen (which gives the chain-carrying peroxy radical, RO 2 ) if this stabilization mechanism is to be important in the inhibition of radiation-induced oxidative degradation of polyolefins by HAS. The rate of this reaction is high and in solution the rate coefficient is from 1 to 9x10 8 l mol - 1 s - 1. The efficient radical trapping by nitroxides has been widely employed in spin-trapping studies by electron spin resonance (esr) spectroscopy]. In addition to the hindered piperidine structure of commercial HAS, more rigid aromatic systems have been studied that are more stable to oxidative degradation and are more efficient at scavenging alkyl radicals. One such family is the iso-indoline nitroxide system, TMDBIO, shown below which, as it contains the phenanthrene fluorophore, is termed phenanthrene nitroxide. This nitroxide only becomes fluorescent when it reacts with alkyl radicals or is reduced and is termed profluorescent. TMDBIO has a vanishingly small fluorescence quantum yield (φ∼10 - 4) due to the enhanced intersystem crossing from the first excited singlet state to the ground state due to electron exchange interactions of the nitroxyl radical. When the nitroxide traps an alkyl radical, R, the resulting alkoxy amine is fluorescent (φ∼10 - 1) and the emission intensity is a measure of the number of reactions that have occurred. This property may be exploited by using quantitative fluorescence spectroscopy to follow the reaction of the nitroxide with alkyl radicals

Free radical reactions of isoxazole and pyrazole derivatives of hispolon: kinetics correlated with molecular descriptors.

Science.gov (United States)

Shaikh, Shaukat Ali M; Barik, Atanu; Singh, Beena G; Modukuri, Ramani V; Balaji, Neduri V; Subbaraju, Gottumukkala V; Naik, Devidas B; Priyadarsini, K Indira

2016-12-01

Hispolon (HS), a natural polyphenol found in medicinal mushrooms, and its isoxazole (HI) and pyrazole (HP) derivatives have been examined for free radical reactions and in vitro antioxidant activity. Reaction of these compounds with one-electron oxidant, azide radicals ([Formula: see text]) and trichloromethyl peroxyl radicals ([Formula: see text]), model peroxyl radicals, studied by nanosecond pulse radiolysis technique, indicated formation of phenoxyl radicals absorbing at 420 nm with half life of few hundred microseconds (μs). The formation of phenoxyl radicals confirmed that the phenolic OH is the active centre for free radical reactions. Rate constant for the reaction of these radicals with these compounds were in the order k HI ≅ k HP  >   k HS . Further the compounds were examined for their ability to inhibit lipid peroxidation in model membranes and also for the scavenging of 2,2'-diphenyl-1-picrylhydrazyl (DPPH) radical and superoxide ([Formula: see text]) radicals. The results suggested that HP and HI are less efficient than HS towards these radical reactions. Quantum chemical calculations were performed on these compounds to understand the mechanism of reaction with different radicals. Lower values of adiabatic ionization potential (AIP) and elevated highest occupied molecular orbital (HOMO) for HI and HP compared with HS controlled their activity towards [Formula: see text] and [Formula: see text] radicals, whereas the contribution of overall anion concentration was responsible for higher activity of HS for DPPH, [Formula: see text], and lipid peroxyl radical. The results confirm the role of different structural moieties on the antioxidant activity of hispolon derivatives.

Free radical reaction characteristics of coal low-temperature oxidation and its inhibition method.

Science.gov (United States)

Li, Zenghua; Kong, Biao; Wei, Aizhu; Yang, Yongliang; Zhou, Yinbo; Zhang, Lanzhun

2016-12-01

Study on the mechanism of coal spontaneous combustion is significant for controlling fire disasters due to coal spontaneous combustion. The free radical reactions can explain the chemical process of coal at low-temperature oxidation. Electron spin resonance (ESR) spectroscopy was used to measure the change rules of the different sorts and different granularity of coal directly; ESR spectroscopy chart of free radicals following the changes of temperatures was compared by the coal samples applying air and blowing nitrogen, original coal samples, dry coal samples, and demineralized coal samples. The fragmentation process was the key factor of producing and initiating free radical reactions. Oxygen, moisture, and mineral accelerated the free radical reactions. Combination of the free radical reaction mechanism, the mechanical fragmentation leaded to the elevated CO concentration, fracturing of coal pillar was more prone to spontaneous combustion, and spontaneous combustion in goaf accounted for a large proportion of the fire in the mine were explained. The method of added diphenylamine can inhibit the self-oxidation of coal effectively, the action mechanism of diphenylamine was analyzed by free radical chain reaction, and this research can offer new method for the development of new flame retardant.

Enantioselective Alkylation of 2-Oxindoles Catalyzed by a Bifunctional Phase-Transfer Catalyst: Synthesis of (-)-Debromoflustramine B.

Science.gov (United States)

Craig, Ryan; Sorrentino, Emiliano; Connon, Stephen J

2018-03-26

A new bifunctional phase-transfer catalyst that employs hydrogen bonding as a control element was developed to promote efficient enantioselective S N 2 reactions for the construction all-carbon quaternary stereocenters in high yield and excellent enantioselectivity (up to 97 % ee) utilizing the alkylation of a malleable oxindole substrate. The utility of the methodology was demonstrated through a concise and highly enantioselective synthesis of (-)-debromoflustramine B. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Application of Phosphine-Phosphite Ligands in the Iridium Catalyzed Enantioselective Hydrogenation of 2-Methylquinoline

Directory of Open Access Journals (Sweden)

Miguel Rubio

2010-10-01

Full Text Available The hydrogenation of 2-methylquinoline with Ir catalysts based on chiral phosphine-phosphites has been investigated. It has been observed that the reaction is very sensitive to the nature of the ligand. Optimization of the catalyst, allowed by the highly modular structure of these phosphine-phosphites, has improved the enantioselectivity of the reaction up to 73% ee. The influence of additives in this reaction has also been investigated. Contrary to the beneficial influence observed in related catalytic systems, iodine has a deleterious effect in the present case. Otherwise, aryl phosphoric acids produce a positive impact on catalyst activity without a decrease on enantioselectivity.

Radical Reactions in the Gas Phase: Recent Development and Application in Biomolecules

Directory of Open Access Journals (Sweden)

Yang Gao

2014-01-01

Full Text Available This review summarizes recent literature describing the use of gas phase radical reactions for structural characterization of complex biomolecules other than peptides. Specifically, chemical derivatization, in-source chemical reaction, and gas phase ion/ion reactions have been demonstrated as effective ways to generate radical precursor ions that yield structural informative fragments complementary to those from conventional collision-induced dissociation (CID. Radical driven dissociation has been applied to a variety of biomolecules including peptides, nucleic acids, carbohydrates, and phospholipids. The majority of the molecules discussed in this review see limited fragmentation from conventional CID, and the gas phase radical reactions open up completely new dissociation channels for these molecules and therefore yield high fidelity confirmation of the structures of the target molecules. Due to the extensively studied peptide fragmentation, this review focuses only on nonpeptide biomolecules such as nucleic acids, carbohydrates, and phospholipids.

Research progress on trifluoromethyl-based radical reaction process

Science.gov (United States)

Song, Hao

2017-12-01

Due to the unique properties imparted by the trifluoromethyl group, such as high electron density and strong lipotropy, which effectively improve acidity, lipophilicity and metabolic stability of the molecule itself, trifluoromethyl-substituted organic compounds are becoming increasingly important as structural motifs in pharmaceuticals, agrochemicals and organic materials. In this review, we present several methods developed for the direct introduction of a trifluoromethyl group, beginning with its rich and storied history. Then the present article addresses mechanism and process in carbon-carbon bond forming reaction based on radical process which is divided into three parts according to the way of CF3 radical generation. Finally, challenges and opportunities of researches on trifluoromethylation reactions facing are prospected.

Reaction kinetics of resveratrol with tert-butoxyl radicals

International Nuclear Information System (INIS)

Džeba, Iva; Pedzinski, Tomasz; Mihaljević, Branka

2012-01-01

The rate constant for the reaction of t-butoxyl radicals with resveratrol was studied under pseudo-first order conditions. The rate constant was determined by measuring the phenoxyl radical formation rate at 390 nm as function of resveratrol concentration in acetonitrile. The rate constant was determined to be 6.5×10 8 M −1 s −1 . This high value indicates the high reactivity consistent with the strong antioxidant activity of resveratrol. - Highlights: ► tert-butoxyl radicals were generated directly using laser flash photolysis. ► Rate constant was determined by the phenoxyl radical formation rate in acetonitrile. ► Rate constant was determined to be 6.5×10 8 M −1 s −1 .

Vancomycin Molecular Interactions: Antibiotic and Enantioselective Mechanisms

Science.gov (United States)

Ward, Timothy J.; Gilmore, Aprile; Ward, Karen; Vowell, Courtney

Medical studies established that vancomycin and other related macrocyclic antibiotics have an enhanced antimicrobial activity when they are associated as dimers. The carbohydrate units attached to the vancomycin basket have an essential role in the dimerization reaction. Covalently synthesized dimers were found active against vancomycin-resistant bacterial strains. A great similarity between antibiotic potential and enantioselectivity was established. A covalent vancomycin dimer was studied in capillary electrophoresis producing excellent chiral separation of dansyl amino acids. Balhimycin is a macrocyclic glycopeptide structurally similar to vancomycin. The small differences are, however, responsible for drastic differences in enantioselectivity in the same experimental conditions. Contributions from studies examining vancomycin's mechanism for antimicrobial activity have substantially aided our understanding of its mechanism in chiral recognition.

Higher-order human telomeric G-quadruplex DNA metalloenzymes enhance enantioselectivity in the Diels-Alder reaction.

Science.gov (United States)

Li, Yinghao; Jia, Guoqing; Wang, Changhao; Cheng, Mingpan; Li, Can

2015-03-02

Short human telomeric (HT) DNA sequences form single G-quadruplex (G4 ) units and exhibit structure-based stereocontrol for a series of reactions. However, for more biologically relevant higher-order HT G4 -DNAs (beyond a single G4 unit), the catalytic performances are unknown. Here, we found that higher-order HT G4 -DNA copper metalloenzymes (two or three G4 units) afford remarkably higher enantioselectivity (>90 % ee) and a five- to sixfold rate increase, compared to a single G4 unit, for the Diels-Alder reaction. Electron paramagnetic resonance (EPR) and enzymatic kinetic studies revealed that the distinct catalytic function between single and higher-order G4 -DNA copper metalloenzymes can be attributed to different Cu(II) coordination environments and substrate specificity. Our finding suggests that, like protein enzymes and ribozymes, higher-order structural organization is crucial for G4 -DNA-based catalysis. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Dibromine radical anion reactions with heme enzymes

International Nuclear Information System (INIS)

Gebicka, L.; Gebicki, J.L.

1996-01-01

Reactions of Br 2 radical anion with heme enzymes, catalase horseradish peroxidase, have been studied by pulse radiolysis. It has been found that Br 2 - does not react with the heme centre of investigated enzymes. Dibromine radical anion reacts with tryptophan residues of catalase without any influence on the activity of catalase. It is suggested that in pulse radiolysis studies, where horseradish peroxidase is at about tenfold excess toward Br 2 - , the enzyme is modified rather by Br 2 , than by Br 2 - . (author). 26 refs., 3 figs

Tuning and Switching Enantioselectivity of Asymmetric Carboligation in an Enzyme through Mutational Analysis of a Single Hot Spot.

Science.gov (United States)

Wechsler, Cindy; Meyer, Danilo; Loschonsky, Sabrina; Funk, Lisa-Marie; Neumann, Piotr; Ficner, Ralf; Brodhun, Florian; Müller, Michael; Tittmann, Kai

2015-12-01

Enantioselective bond making and breaking is a hallmark of enzyme action, yet switching the enantioselectivity of the reaction is a difficult undertaking, and typically requires extensive screening of mutant libraries and multiple mutations. Here, we demonstrate that mutational diversification of a single catalytic hot spot in the enzyme pyruvate decarboxylase gives access to both enantiomers of acyloins acetoin and phenylacetylcarbinol, important pharmaceutical precursors, in the case of acetoin even starting from the unselective wild-type protein. Protein crystallography was used to rationalize these findings and to propose a mechanistic model of how enantioselectivity is controlled. In a broader context, our studies highlight the efficiency of mechanism-inspired and structure-guided rational protein design for enhancing and switching enantioselectivity of enzymatic reactions, by systematically exploring the biocatalytic potential of a single hot spot. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kinetics and mechanisms of reactions of CF, CHF, and CF2 radicals

International Nuclear Information System (INIS)

Hsu, D.S.Y.; Umstead, M.E.; Lin, M.C.

1978-01-01

This chapter reviews briefly methods for the production of CF, CHF and CF 2 , and inmore detail, the reactions of these interesting and important radicals. Although a considerable, but not extensive, amount of work has been done on the reactions of CF 2 , little of the chemistry of CF and CHF is known. This chapter also includes the preliminary results of some experiments carried out in this Laboratory on the dynamics of some of the reactions involving these radicals. These results were largely arrived at through investigations of the degree of vibrational excitation of the HF and CO reaction products, determined by HF and CO laser emission and CO laser resonance absorption measurements. The coverage of this review is restricted to the gas phase chemistry of these radicals, and does not include their addition reactions to olefins

Enantioselective biotransformations of nitriles in organic synthesis.

Science.gov (United States)

Wang, Mei-Xiang

2015-03-17

The hydration and hydrolysis of nitriles are valuable synthetic methods used to prepare carboxamides and carboxylic acids. However, chemical hydration and hydrolysis of nitriles involve harsh reaction conditions, have low selectivity, and generate large amounts of waste. Therefore, researchers have confined the scope of these reactions to simple nitrile substrates. However, biological transformations of nitriles are highly efficient, chemoselective, and environmentally benign, which has led synthetic organic chemists and biotechologists to study these reactions in detail over the last two decades. In nature, biological systems degrade nitriles via two distinct pathways: nitrilases catalyze the direct hydrolysis of nitriles to afford carboxylic acids with release of ammonia, and nitrile hydratases catalyze the conversion of nitriles into carboxamides, which then furnish carboxylic acids via hydrolysis in the presence of amidases. Researchers have subsequently developed biocatalytic methods into useful industrial processes for the manufacture of commodity chemicals, including acrylamide. Since the late 1990s, research by my group and others has led to enormous progress in the understanding and application of enantioselective biotransformations of nitriles in organic synthesis. In this Account, I summarize the important advances in enantioselective biotransformations of nitriles and amides, with a primary focus on research from my laboratory. I describe microbial whole-cell-catalyzed kinetic resolution of various functionalized nitriles, amino- and hydroxynitriles, and nitriles that contain small rings and the desymmetrization of prochiral and meso dinitriles and diamides. I also demonstrate how we can apply the biocatalytic protocol to synthesize natural products and bioactive compounds. These nitrile biotransformations offer an attractive and unique protocol for the enantioselective synthesis of polyfunctionalized organic compounds that are not readily obtainable by

Iron-chelating agents never suppress Fenton reaction but participate in quenching spin-trapped radicals

International Nuclear Information System (INIS)

Li Linxiang; Abe, Yoshihiro; Kanagawa, Kiyotada; Shoji, Tomoko; Mashino, Tadahiko; Mochizuki, Masataka; Tanaka, Miho; Miyata, Naoki

2007-01-01

Hydroxyl radical formation by Fenton reaction in the presence of an iron-chelating agent such as EDTA was traced by two different assay methods; an electron spin resonance (ESR) spin-trapping method with 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and high Performance liquid chromatography (HPLC)-fluorescence detection with terephthalic acid (TPA), a fluorescent probe for hydroxyl radicals. From the ESR spin-trapping measurement, it was observed that EDTA seemed to suppress hydroxyl radical formation with the increase of its concentration. On the other hand, hydroxyl radical formation by Fenton reaction was not affected by EDTA monitored by HPLC assay. Similar inconsistent effects of other iron-chelating agents such as nitrylotriacetic acid (NTA), diethylenetriamine penta acetic acid (DTPA), oxalate and citrate were also observed. On the addition of EDTA solution to the reaction mixture 10 min after the Fenton reaction started, when hydroxyl radical formation should have almost ceased but the ESR signal of DMPO-OH radicals could be detected, it was observed that the DMPO-OH· signal disappeared rapidly. With the simultaneous addition of Fe(II) solution and EDTA after the Fenton reaction ceased, the DMPO-OH· signal disappeared more rapidly. The results indicated that these chelating agents should enhance the quenching of [DMPO-OH]· radicals by Fe(II), but they did not suppress Fenton reaction by forming chelates with iron ions

Standard Gibbs free energies of reactions of ozone with free radicals in aqueous solution: quantum-chemical calculations.

Science.gov (United States)

Naumov, Sergej; von Sonntag, Clemens

2011-11-01

Free radicals are common intermediates in the chemistry of ozone in aqueous solution. Their reactions with ozone have been probed by calculating the standard Gibbs free energies of such reactions using density functional theory (Jaguar 7.6 program). O(2) reacts fast and irreversibly only with simple carbon-centered radicals. In contrast, ozone also reacts irreversibly with conjugated carbon-centered radicals such as bisallylic (hydroxycylohexadienyl) radicals, with conjugated carbon/oxygen-centered radicals such as phenoxyl radicals, and even with nitrogen- oxygen-, sulfur-, and halogen-centered radicals. In these reactions, further ozone-reactive radicals are generated. Chain reactions may destroy ozone without giving rise to products other than O(2). This may be of importance when ozonation is used in pollution control, and reactions of free radicals with ozone have to be taken into account in modeling such processes.

Asymmetric Synthesis of Optically Active Spirocyclic Indoline Scaffolds through an Enantioselective Reduction of Indoles

KAUST Repository

Borrmann, Ruediger

2016-11-30

An enantioselective synthesis of spirocyclic indoline scaffolds was achieved by applying an asymmetric iridium-catalyzed hydrogenation of 3H-indoles. Low catalyst loadings and mild reaction conditions provide a broad range of differently substituted products with excellent yields and enantioselectivities. The developed methodology allows an efficient synthesis of this important spirocyclic structural motif, which is present in numerous biologically active molecules and privileged structures in medicinal chemistry.

Ultrasound-Assisted Enantioselective Esterification of Ibuprofen Catalyzed by a Flower-Like Nanobioreactor

Directory of Open Access Journals (Sweden)

Baiyi An

2016-04-01

Full Text Available A flower-like nanobioreactor was prepared for resolution of ibuprofen in organic solvents. Ultrasound irradiation has been used to improve the enzyme performance of APE1547 (a thermophilic esterase from the archaeon Aeropyrum pernix K1 in the enantioselective esterification. Under optimum reaction conditions (ultrasound power, 225 W; temperature, 45 °C; water activity, 0.21, the immobilized APE1547 showed an excellent catalytic performance (enzyme activity, 13.26 μmol/h/mg; E value, 147.1. After ten repeated reaction batches, the nanobioreactor retained almost 100% of its initial enzyme activity and enantioselectivity. These results indicated that the combination of the immobilization method and ultrasound irradiation can enhance the enzyme performance dramatically.

Atmospheric chemistry of CF3O radicals: Reaction with H2O

DEFF Research Database (Denmark)

Wallington, T.J.; Hurley, M.D.; Schneider, W.F.

1993-01-01

Evidence is presented that CF3O radicals react with H2O in the gas phase at 296 K to give CF3OH and OH radicals. This reaction is calculated to be exothermic by 1.7 kcal mol-I implying a surprisingly strong CF3O-H bond energy of 120 +/- 3 kcal mol-1. Results from a relative rate experimental study...... suggest that the rate constant for the reaction of CF3O radicals with H2O lies in the range (0.2-4.0) X 10(-17) cm3 molecule-1 s-1. Implications for the atmospheric chemistry of CF3O radicals are discussed....

Free radical reactions of daunorubicin

International Nuclear Information System (INIS)

Houee-Levin, C.

1991-01-01

Daunorubicin is an antitumor antibiotic activated in vivo by reduction. Its mechanism of action involves DNA and topoisomerase attack, but side effects are cytotoxicity related to free radical formation. Therefore the mechanism of the one-electron reduction of the drug and the reactions of the daunorubicin transients towards compounds of biological interest have been studied by the methods of radiolysis, in order to provide possible explanations of the drug mechanism of action. Their relative importance in cellular conditions is discussed [fr

Thermodynamic and kinetic analysis of the reaction between biological catecholamines and chlorinated methylperoxy radicals

Science.gov (United States)

Dimić, Dušan S.; Milenković, Dejan A.; Marković, Jasmina M. Dimitrić; Marković, Zoran S.

2018-05-01

The antiradical potency of catecholamines (dopamine, epinephrine, norepinephrine, L-DOPA), metabolites of dopamine (homovanillic acid, 3-methoxytyramine and 3,4-dihydroxyphenylacetic acid) and catechol towards substituted methylperoxy radicals is investigated. The thermodynamic parameters, together with the kinetic approach, are used to determine the most probable mechanism of action. The natural bond orbital and quantum theory of atoms in molecules are utilised to explain the highest reactivity of trichloromethylperoxy radical. The preferred mechanism is dependent both on the thermodynamic and kinetic parameters . The number of chlorine atoms on radical, the presence of intra-molecular hydrogen bond and number of hydroxy groups attached to the aromatic ring significantly influence the mechanism. The results suggest that sequential proton loss electron transfer (SPLET) is the most probable for reaction with methylperoxy and hydrogen atom transfer (HAT) for reaction with trichloromethylperoxy radicals, with a gradual transition between SPLET and HAT for other two radicals. Due to the significant deprotonation of molecules containing the carboxyl group, the respective anions are also investigated. The HAT and SPLET mechanisms are highly competitive in reaction with MP radical, while the dominant mechanism towards chlorinated radicals is HAT. The reactions in methanol and benzene are also discussed.

Rate constants for the reactions of free radicals with oxygen in solution

International Nuclear Information System (INIS)

Maillard, B.; Ingold, K.U.; Scaiano, J.C.

1983-01-01

The kinetics of the rections of several free radicals with oxygen have been examined in solution at 300 K using laser flash photolysis techniques. The reactions of resonance-stabilized radicals are only slightly slower than those of nonstabilized radicals: for example, for tert-butyl (in cyclohexane), 4.93 x 10 9 ; benzyl, 2.36 x 10 9 (in cyclohexane); cyclohexadienyl (in benzene), 1.64 x 10 9 M -1 s -1 . The reaction of butyl-tin (n-Bu 3 Sn.) radicals is unusually fast (7.5 x 10 9 M -1 s -1 ), a fact that has been tentatively attributed to a relaxation of spin selection rules due to heavy atom effects. 1 table

Studying mechanism of radical reactions: From radiation to nitroxides as research tools

Science.gov (United States)

Maimon, Eric; Samuni, Uri; Goldstein, Sara

2018-02-01

Radicals are part of the chemistry of life, and ionizing radiation chemistry serves as an indispensable research tool for elucidation of the mechanism(s) underlying their reactions. The ever-increasing understanding of their involvement in diverse physiological and pathological processes has expanded the search for compounds that can diminish radical-induced damage. This review surveys the areas of research focusing on radical reactions and particularly with stable cyclic nitroxide radicals, which demonstrate unique antioxidative activities. Unlike common antioxidants that are progressively depleted under oxidative stress and yield secondary radicals, nitroxides are efficient radical scavengers yielding in most cases their respective oxoammonium cations, which are readily reduced back in the tissue to the nitroxide thus continuously being recycled. Nitroxides, which not only protect enzymes, cells, and laboratory animals from diverse kinds of biological injury, but also modify the catalytic activity of heme enzymes, could be utilized in chemical and biological systems serving as a research tool for elucidating mechanisms underlying complex chemical and biochemical processes.

Unusual spin-trap chemistry for the reaction of hydroxyl radical with the carcinogen N-nitrosodimethylamine

Science.gov (United States)

Wink, David A.; Desrosiers, Marc F.

The reaction of the potent carcinogen N-nitrosodimethylamine (NDMA) with hydroxyl radical generated via radiolysis was studied using EPR techniques. Attempts to spin trap NDMA radical intermediates with 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS) produced only unusual DBNBS radicals. One of these radicals was shown to be generated by both reaction of DBNBS with nitric oxide, and direct oxidation of DBNBS with an inorganic oxidant ( .Br -2). Another DBNBS radical was identified as a sulfite spin adduct resulting from the degradation of DBNBS by a NDMA reactive intermediate. In the absence of DBNBS, hydroxyl radical reaction with NDMA gave the dimethylnitroxide radical. Unexpectedly, addition of DBNBS to a solution containing dimethylnitroxide produced an EPR spectrum nearly identical to that of NDMA solutions with DBNBS added before radiolysis. A proposed mechanism accounting for these observations is presented.

Rhodium-catalyzed enantioselective intramolecular C-H silylation for the syntheses of planar-chiral metallocene siloles.

Science.gov (United States)

Zhang, Qing-Wei; An, Kun; Liu, Li-Chuan; Yue, Yuan; He, Wei

2015-06-01

Reported herein is the rhodium-catalyzed enantioselective C-H bond silylation of the cyclopentadiene rings in Fe and Ru metallocenes. Thus, in the presence of (S)-TMS-Segphos, the reactions took place under very mild conditions to afford metallocene-fused siloles in good to excellent yields and with ee values of up to 97%. During this study it was observed that the steric hindrance of chiral ligands had a profound influence on the reactivity and enantioselectivity of the reaction, and might hold the key to accomplishing conventionally challenging asymmetric C-H silylations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kinetics of elementary atom and radical reactions: Progress report

International Nuclear Information System (INIS)

Gordon, R.J.

1986-01-01

Our research program is concerned with the kinetics of elementary gas phase reactions and energy transfer involving polyatomic molecules. We report here on three ongoing projects: The reaction of oxygen atoms with hydrogen molecules, the electronic relaxation of NH radicals, and the vibrational relaxation of highly excited SF 6 molecules. 10 refs., 5 figs

Enantioselective Construction of 3-Hydroxypiperidine Scaffolds by Sequential Action of Light and Rhodium upon N-Allylglyoxylamides.

Science.gov (United States)

Ishida, Naoki; Nečas, David; Masuda, Yusuke; Murakami, Masahiro

2015-06-15

3-Hydroxypiperidine scaffolds were enantioselectively constructed in an atom-economical way by sequential action of light and rhodium upon N-allylglyoxylamides. In a formal sense, the allylic C-H bond was selectively cleaved and enantioselectively added across the ketonic carbonyl group with migration of the double bond (carbonyl-ene-type reaction). © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Unusual spin-trap chemistry for the reaction of hydroxyl radical with the carcinogen N-nitrosodimethylamine

Energy Technology Data Exchange (ETDEWEB)

Wink, D A [National Cancer Inst., Frederick, MD (United States); Desrosiers, M F [National Inst. of Standards and Technology, Gaithersburg, MD (United States)

1991-01-01

The reaction of the potent carcinogen N-nitrosodimethylamine (NDMA) with hydroxyl radical generated via radiolysis was studied using EPR techniques. Attempts to spin trap NDMA radical intermediates with 3.5-dibromo-4-nitrosobenzene sulfonate (DBNBS) produced only unusual DBNBS radicals. One of these radicals was shown to be generated by both reaction of DBNBS with nitric oxide, and direct oxidation of DBNBS with an inorganic oxidant (BR{sub 2}{sup -}). Another DBNBS radical was identified as a sulfite spin adduct resulting from the degradation of DBNBS by a NDMA reactive inter-mediate. In the absence of DBNBS, hydroxyl radical reaction with NDMA gave the dimethylnitroxide produced an EPR spectrum nearly identical to that of NDMA solutions with DBNBS added before radiolysis. A proposed mechanism accounting for these observations is presented. (author).

Unusual spin-trap chemistry for the reaction of hydroxyl radical with the carcinogen N-nitrosodimethylamine

International Nuclear Information System (INIS)

Wink, D.A.; Desrosiers, M.F.

1991-01-01

The reaction of the potent carcinogen N-nitrosodimethylamine (NDMA) with hydroxyl radical generated via radiolysis was studied using EPR techniques. Attempts to spin trap NDMA radical intermediates with 3.5-dibromo-4-nitrosobenzene sulfonate (DBNBS) produced only unusual DBNBS radicals. One of these radicals was shown to be generated by both reaction of DBNBS with nitric oxide, and direct oxidation of DBNBS with an inorganic oxidant (BR 2 - ). Another DBNBS radical was identified as a sulfite spin adduct resulting from the degradation of DBNBS by a NDMA reactive inter-mediate. In the absence of DBNBS, hydroxyl radical reaction with NDMA gave the dimethylnitroxide produced an EPR spectrum nearly identical to that of NDMA solutions with DBNBS added before radiolysis. A proposed mechanism accounting for these observations is presented. (author)

Stepwise radical cation Diels-Alder reaction via multiple pathways.

Science.gov (United States)

Shimizu, Ryo; Okada, Yohei; Chiba, Kazuhiro

2018-01-01

Herein we disclose the radical cation Diels-Alder reaction of aryl vinyl ethers by electrocatalysis, which is triggered by an oxidative SET process. The reaction clearly proceeds in a stepwise fashion, which is a rare mechanism in this class. We also found that two distinctive pathways, including "direct" and "indirect", are possible to construct the Diels-Alder adduct.

Reaction kinetics and mechanisms of organosilicon fungicide flusilazole with sulfate and hydroxyl radicals.

Science.gov (United States)

Mercado, D Fabio; Bracco, Larisa L B; Arques, Antonio; Gonzalez, Mónica C; Caregnato, Paula

2018-01-01

Flusilazole is an organosilane fungicide used for treatments in agriculture and horticulture for control of diseases. The reaction kinetics and mechanism of flusilazole with sulfate and hydroxyl radicals were studied. The rate constant of the radicals with the fungicide were determined by laser flash photolysis of peroxodisulfate and hydrogen peroxide. The results were 2.0 × 10 9 s -1 M -1 for the reaction of the fungicide with HO and 4.6 × 10 8  s -1  M -1 for the same reaction with SO 4 - radicals. The absorption spectra of organic intermediates detected by laser flash photolysis of S 2 O 8 2- with flusilazole, were identified as α-aminoalkyl and siloxyl radicals and agree very well with those estimated employing the time-dependent density functional theory with explicit account for bulk solvent effects. In the continuous photolysis experiments, performed by photo-Fenton reaction of the fungicide, the main degradation products were: (bis(4-fluorophenyl)-hydroxy-methylsilane) and the non-toxic silicic acid, diethyl bis(trimethylsilyl) ester, in ten and twenty minutes of reaction, respectively. Copyright © 2017. Published by Elsevier Ltd.

Rate constant for reaction of hydroxyl radicals with bicarbonate ions

International Nuclear Information System (INIS)

Buxton, G.V.; Elliot, A.J.

1986-01-01

The rate constant for reaction of hydroxyl radicals with the bicarbonate ion has been determined to be 8.5 x 10 6 dm 3 mol -1 s -1 . This value was calculated from: the measured rate of formation of the CO 3 - radical in pulsed electron irradiation of bicarbonate solutions over the pH range 7.0 to 9.4; the pK for the equilibrium HCO 3 - = CO 3 2- + H + ; and the rate constant for hydroxyl radicals reacting with the carbonate ion. (author)

Enantioselective Evans-Tishchenko Reduction of b-Hydroxyketone Catalyzed by Lithium Binaphtholate

Directory of Open Access Journals (Sweden)

Makoto Nakajima

2011-06-01

Full Text Available Lithium diphenylbinaphtholate catalyzed the enantioselective Evans-Tishchenko reduction of achiral b-hydroxyketones to afford monoacyl-protected 1,3-diols with high stereoselectivities. In the reaction of racemic b-hydroxyketones, kinetic optical resolution occurred in a highly stereoselective manner.

High-level theoretical characterization of the vinoxy radical (•CH2CHO) + O2 reaction

Science.gov (United States)

Weidman, Jared D.; Allen, Ryan T.; Moore, Kevin B.; Schaefer, Henry F.

2018-05-01

Numerous processes in atmospheric and combustion chemistry produce the vinoxy radical (•CH2CHO). To understand the fate of this radical and to provide reliable energies needed for kinetic modeling of such processes, we have examined its reaction with O2 using highly reliable theoretical methods. Utilizing the focal point approach, the energetics of this reaction and subsequent reactions were obtained using coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] extrapolated to the complete basis set limit. These extrapolated energies were appended with several corrections including a treatment of full triples and connected quadruple excitations, i.e., CCSDT(Q). In addition, this study models the initial vinoxy radical + O2 reaction for the first time with multireference methods. We predict a barrier for this reaction of approximately 0.4 kcal mol-1. This result agrees with experimental findings but is in disagreement with previous theoretical studies. The vinoxy radical + O2 reaction produces a 2-oxoethylperoxy radical which can undergo a number of unimolecular reactions. Abstraction of a β-hydrogen (a 1,4-hydrogen shift) and dissociation back to reactants are predicted to be competitive to each other due to their similar barriers of 21.2 and 22.3 kcal mol-1, respectively. The minimum-energy β-hydrogen abstraction pathway produces a hydroperoxy radical (QOOH) that eventually decomposes to formaldehyde, CO, and •OH. Two other unimolecular reactions of the peroxy radical are α-hydrogen abstraction (38.7 kcal mol-1 barrier) and HO2• elimination (43.5 kcal mol-1 barrier). These pathways lead to glyoxal + •OH and ketene + HO2• formation, respectively, but they are expected to be uncompetitive due to their high barriers.

Structure reactivity relationship in the reaction of DNA guanyl radicals with hydroxybenzoates

Energy Technology Data Exchange (ETDEWEB)

Do, Trinh T.; Tang, Vicky J.; Aguilera, Joseph A. [Department of Radiology University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0610 (United States); Milligan, Jamie R., E-mail: jmilligan@ucsd.ed [Department of Radiology University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0610 (United States)

2010-11-15

In DNA, guanine bases are the sites from which electrons are most easily removed. As a result of hole migration to this stable location on guanine, guanyl radicals are major intermediates in DNA damage produced by the direct effect of ionizing radiation (ionization of the DNA itself and not through the intermediacy of water radicals). We have modeled this process by employing gamma irradiation in the presence of thiocyanate ions, a method which also produces single electron oxidized guanyl radicals in plasmid DNA in aqueous solution. The stable products formed in DNA from these radicals are detected as strand breaks after incubation with the FPG protein. When a phenolic compound is present in the solution during gamma irradiation, the formation of guanyl radical species is decreased by electron donation from the phenol to the guanyl radical. We have quantified the rate of this reaction for four different phenolic compounds bearing carboxylate substituents as proton acceptors. A comparison of the rates of these reactions with the redox strengths of the phenolic compounds reveals that salicylate reacts ca. 10-fold faster than its structural analogs. This observation is consistent with a reaction mechanism involving a proton coupled electron transfer, because intra-molecular transfer of a proton from the phenolic hydroxyl group to the carboxylate group is possible only in salicylate, and is favored by the strong 6-membered ring intra-molecular hydrogen bond in this compound.

Radical reactions in vivo - an overview

International Nuclear Information System (INIS)

Saran, M.; Bors, W.

1990-01-01

Generation of radicals in vivo depends on metabolic activities. The reactions are usually influenced by (i) the presence and concentration of oxygen, (ii) the availability of transition metals (effects of binding and compartimentalization), (iii) the level of reductants and antioxidants (e.g. nutritional effects). The effects of radicals are thought to be due to (i) membrane damage (affecting passive or active transport through altered fluidity/function interrelationships, intercellular messenging through modifications in the synthesis of prostaglandins and leukotrienes); (ii) protein damage (e.g. affecting membrane transporters, channel proteins, receptor or regulatory proteins, immunomodulators); (iii) damage to DNA. Defense mechanisms consist of (i) prevention of the 'spreading' of primary damage by low molecular weight antioxidants (e.g. vitamin E, GSH, vitamin C, β-carotene, uric acid); (ii) prevention or limitation of 'secondary' damage by enzymes (e.g. GSH-peroxidase, catalase, superoxide dismutase, DT-diaphorase) and/or chelators; (iii) repair processes, e.g. lipid degradation/membrane repair enzymes (phospholipases, peroxidases, some transferases and reductases), protein disposal or repair enzymes (proteases, GSSG-reductase), DNA degradation or repair enzymes (exonucleases III, endonucleases III and IV, glycosylases, polymerases). Recent hypotheses on a messenging function of the superoxide anion O 2 - are discussed and possible implications of cross-reactions between O 2 - and nitric oxide (endothelium-derived relaxing factor EDRF) are shortly mentioned. (orig.)

Optimisation of stabilised carboxylesterase NP for enantioselective hydrolysis of naproxen methyl ester

CSIR Research Space (South Africa)

Steenkamp, Lucia H

2008-12-01

Full Text Available Although the enantioselective kinetic resolution of ester racemates of the non-steroidal antiinflammatory drug naproxen ([2-(6-methoxy-2-naphthyl) propionic acid]) is a common demonstration for biocatalysis, the enantiomeric excess of the reactions...

Ab initio molecular dynamics of the reaction of quercetin with superoxide radical

International Nuclear Information System (INIS)

Lespade, Laure

2016-01-01

Highlights: • Ab initio molecular dynamics is performed to describe the reaction of quercetin and superoxide. • The reaction occurs near the sites 4′ and 7 when the system contains sufficiently water molecules. • The difference of reactivity of superoxide compared to commonly used radicals as DPPH · or ABTS ·+ is explained. - Abstract: Superoxide plays an important role in biology but in unregulated concentrations it is implicated in a lot of diseases such as cancer or atherosclerosis. Antioxidants like flavonoids are abundant in plant and are good scavengers of superoxide radical. The modeling of superoxide scavenging by flavonoids from the diet still remains a challenge. In this study, ab initio molecular dynamics of the reaction of the flavonoid quercetin toward superoxide radical has been carried out using Car–Parrinello density functional theory. The study has proven different reactant solvation by modifying the number of water molecules surrounding superoxide. The reaction consists in the gift of a hydrogen atom of one of the hydroxyl groups of quercetin to the radical. When it occurs, it is relatively fast, lower than 100 fs. Calculations show that it depends largely on the environment of the hydroxyl group giving its hydrogen atom, the geometry of the first water layer and the presence of a certain number of water molecules in the second layer, indicating a great influence of the solvent on the reactivity.

Enantioselective Addition of Allyltin Reagents to Amino Aldehydes Catalyzed with Bis(oxazolinylphenylrhodium(III Aqua Complexes

Directory of Open Access Journals (Sweden)

Hisao Nishiyama

2011-06-01

Full Text Available Bis(oxazolinylphenylrhodium(III aqua complexes, (PheboxRhX2(H2O [X = Cl, Br], were found to be efficient Lewis acid catalysts for the enantioselective addition of allyl- and methallyltributyltin reagents to amino aldehydes. The reactions proceed smoothly in the presence of 5–10 mol % of (PheboxRhX2(H2O complex at ambient temperature to give the corresponding amino alcohols with modest to good enantioselectivity (up to 94% ee.

Spin-selective recombination reactions of radical pairs: Experimental test of validity of reaction operators

Energy Technology Data Exchange (ETDEWEB)

Maeda, Kiminori [Department of Chemistry, University of Oxford, Centre for Advanced Electron Spin Resonance, Inorganic Chemistry Laboratory, Oxford (United Kingdom); Liddell, Paul; Gust, Devens [Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, 85287-1604 (United States); Hore, P. J. [Department of Chemistry, University of Oxford, Physical and Theoretical Chemistry Laboratory, Oxford (United Kingdom)

2013-12-21

Spin-selective reactions of radical pairs are conventionally modelled using an approach that dates back to the 1970s [R. Haberkorn, Mol. Phys. 32, 1491 (1976)]. An alternative approach based on the theory of quantum measurements has recently been suggested [J. A. Jones and P. J. Hore, Chem. Phys. Lett. 488, 90 (2010)]. We present here the first experimental attempt to discriminate between the two models. Pulsed electron paramagnetic resonance spectroscopy has been used to investigate intramolecular electron transfer in the radical pair form of a carotenoid-porphyrin-fullerene molecular triad. The rate of spin-spin relaxation of the fullerene radical in the triad was found to be inconsistent with the quantum measurement description of the spin-selective kinetics, and in accord with the conventional model when combined with spin-dephasing caused by rotational modulation of the anisotropic g-tensor of the fullerene radical.

Selective free radical reactions using supercritical carbon dioxide.

Science.gov (United States)

Cormier, Philip J; Clarke, Ryan M; McFadden, Ryan M L; Ghandi, Khashayar

2014-02-12

We report herein a means to modify the reactivity of alkenes, and particularly to modify their selectivity toward reactions with nonpolar reactants (e.g., nonpolar free radicals) in supercritical carbon dioxide near the critical point. Rate constants for free radical addition of the light hydrogen isotope muonium to ethylene, vinylidene fluoride, and vinylidene chloride in supercritical carbon dioxide are compared over a range of pressures and temperatures. Near carbon dioxide's critical point, the addition to ethylene exhibits critical speeding up, while the halogenated analogues display critical slowing. This suggests that supercritical carbon dioxide as a solvent may be used to tune alkene chemistry in near-critical conditions.

A chiral Brønsted acid-catalyzed highly enantioselective Mannich-type reaction of α-diazo esters with in situ generated N-acyl ketimines.

Science.gov (United States)

Unhale, Rajshekhar A; Sadhu, Milon M; Ray, Sumit K; Biswas, Rayhan G; Singh, Vinod K

2018-04-03

A chiral phosphoric acid-catalyzed asymmetric Mannich-type reaction of α-diazo esters with in situ generated N-acyl ketimines, derived from 3-hydroxyisoindolinones has been demonstrated in this communication. A variety of isoindolinone-based α-amino diazo esters bearing a quaternary stereogenic center were afforded in high yields (up to 99%) with excellent enantioselectivities (up to 99% ee). Furthermore, the synthetic utility of the products has been depicted by the hydrogenation of the diazo moiety of adducts.

Coherent chemical kinetics as quantum walks. II. Radical-pair reactions in Arabidopsis thaliana

Science.gov (United States)

Chia, A.; Górecka, A.; Kurzyński, P.; Paterek, T.; Kaszlikowski, D.

2016-03-01

We apply the quantum-walk approach proposed in the preceding paper [A. Chia et al., preceding paper, Phys. Rev. E 93, 032407 (2016), 10.1103/PhysRevE.93.032407] to a radical-pair reaction where realistic estimates for the intermediate transition rates are available. The well-known average hitting time from quantum walks can be adopted as a measure of how quickly the reaction occurs and we calculate this for varying degrees of dephasing in the radical pair. The time for the radical pair to react to a product is found to be independent of the amount of dephasing introduced, even in the limit of no dephasing where the transient population dynamics exhibits strong coherent oscillations. This can be seen to arise from the existence of a rate-limiting step in the reaction and we argue that in such examples, a purely classical model based on rate equations can be used for estimating the time scale of the reaction but not necessarily its population dynamics.

Radical-mediated annulation reactions. A versatile strategy for the preparation of a series of carbocycles.

Science.gov (United States)

Sibi, M P; Chen, J; Rheault, T R

2001-11-15

[reaction--see text] A series of novel 6-endo [4 + 2] and 7-endo [5 + 2] radical-mediated annulation reactions are described. These annulation sequences involve an intermolecular radical addition followed by intramolecular trapping with an allyltin moiety incorporated into the radical precursor fragment. This methodology allows for access to functionalized 6- and 7-membered carbocycles as well as bicyclic compounds with good to excellent levels of stereocontrol.

Computer-Assisted Design and Synthetic Applications of Chiral Enol Borinates: Novel, Highly Enantioselective Aldol Reagents

Directory of Open Access Journals (Sweden)

Gennari Cesare

1998-01-01

Full Text Available We have recently described the development of a quantitative transition state model for the prediction of stereoselectivity in the boron-mediated aldol reaction. This model provides qualitative insights into the factors contributing to the stereochemical outcome of a variety of reactions of synthetic importance. The force field model was used to assist the design and preparation of new chiral boron ligands derived from menthone. The chiral boron enolates were employed in various stereoselective processes, including the addition to chiral aldehydes and the reagent-controlled total synthesis of (3S,4S-statine. The chiral enolates derived from alpha-halo and alpha-oxysubstituted thioacetates were added to aldehydes and imines. Addition to imines leads to the enantioselective synthesis of chiral aziridines, a formal total synthesis of (+-thiamphenicol, and a new highly efficient synthesis of the paclitaxel (taxol® C-13 side-chain and taxol semisynthesis from baccatin III. The stereochemical outcome of the addition to imines was rationalised with the aid of computational studies. Enantioselective addition reactions of the chiral boron enolate derived from thioacetate have successfully been applied to solid phase bound aldehydes to give aldol products in comparable yields and enantioselectivities to the usual solution conditions.

Spin polarization and magnetic effects in radical reactions

International Nuclear Information System (INIS)

Salikhov, K.M.; Molin, Yu.N.; Sagdeev, R.Z.; Buchachenko, A.L.

1984-01-01

Studies on the effects of chemically induced dynamic nuclear and electron polarizations (CIDNP and CIDEP), and magnetic effects in radical reactions, have given rise to a new rapidly-progressing field of chemical physics. It came into being about ten years ago and has been attracting the ever-growing attention of researchers in related areas. The present book is a fairly all-embracing review of the state of affairs in this field. The book presents the physical background (both theoretical and experimental) of CIDNP and CIDEP, of the effects of an external magnetic field and magnetic nuclear moment (magnetic isotope effects) on radical reactions in solutions. Great attention has been paid to the application of chemical spin polarization and magnetic effects to solving various problems of chemical kinetics, structural chemistry, molecular physics, magnetobiology, and radiospectroscopy. The book will be useful for physicists, chemists and biologists employing CIDNP, CIDEP and magnetic effects in their investigations, as well as for researchers in related fields of chemical physics. The book can be also recommended for postgraduates and senior undergraduate students. (Auth.)

Kinetics of several oxygenated carbon-centered free radical reactions with NO2.

Science.gov (United States)

Rissanen, Matti P; Arppe, Suula L; Timonen, Raimo S

2013-05-16

Five oxygenated carbon-centered free radical reactions with nitrogen dioxide (NO2) have been studied in direct time-resolved measurements. Experiments were conducted in a temperature-controlled flow tube reactor coupled to a 193 nm exciplex laser photolysis and a resonance gas lamp photoionization mass spectrometer. Reactions were investigated under pseudofirst-order conditions, with the NO2 concentrations of the experiments in great excess over the initial radical concentrations ([R]0 CH3CO radical reactions with NO2 and, hence, includes the three smallest hydroxyalkyl radical species (CH2OH, CH2CH2OH, and CH3CHOH). The obtained rate coefficients are high with the temperature-dependent rate coefficients given by a formula k(T) = k300K × (T/300 K)(-n) as (in units of cm(3) molecule(-1) s(-1)): k(CH2OH + NO2) = (8.95 ± 2.70) × 10(-11) × (T/300 K)(-0.54±0.27) (T = 298-363 K), k(CH2CH2OH + NO2) = (5.99 ± 1.80) × 10(-11) × (T/300 K)(-1.49±0.45)(T = 241-363 K), k(CH3CHOH + NO2) = (7.48 ± 2.24) × 10(-11) × (T/300 K)(-1.36±0.41) (T = 266-363 K), k(CH3OCH2 + NO2) = (7.85 ± 2.36) × 10(-11) × (T/300 K)(-0.93±0.28) (T = 243-363 K), and k(CH3CO + NO2) = (2.87 ± 0.57) × 10(-11) × (T/300 K)(-2.45±0.49) (T = 241-363 K), where the uncertainties refer to the estimated overall uncertainties of the values obtained. The determined rate coefficients show negative temperature dependence with no apparent bath gas pressure dependence under the current experimental conditions (241-363 K and about 1-3 Torr helium). This behavior is typical for a radical-radical addition mechanism with no potential energy barrier above the energy of the separated reactants in the entrance channel of the reaction. Unfortunately the absence of detected product signals prevented gaining deeper insight into the reaction mechanism.

Ab initio molecular dynamics of the reaction of quercetin with superoxide radical

Energy Technology Data Exchange (ETDEWEB)

Lespade, Laure, E-mail: l.lespade@ism.u-bordeaux1.fr

2016-08-22

Highlights: • Ab initio molecular dynamics is performed to describe the reaction of quercetin and superoxide. • The reaction occurs near the sites 4′ and 7 when the system contains sufficiently water molecules. • The difference of reactivity of superoxide compared to commonly used radicals as DPPH{sup ·} or ABTS{sup ·+} is explained. - Abstract: Superoxide plays an important role in biology but in unregulated concentrations it is implicated in a lot of diseases such as cancer or atherosclerosis. Antioxidants like flavonoids are abundant in plant and are good scavengers of superoxide radical. The modeling of superoxide scavenging by flavonoids from the diet still remains a challenge. In this study, ab initio molecular dynamics of the reaction of the flavonoid quercetin toward superoxide radical has been carried out using Car–Parrinello density functional theory. The study has proven different reactant solvation by modifying the number of water molecules surrounding superoxide. The reaction consists in the gift of a hydrogen atom of one of the hydroxyl groups of quercetin to the radical. When it occurs, it is relatively fast, lower than 100 fs. Calculations show that it depends largely on the environment of the hydroxyl group giving its hydrogen atom, the geometry of the first water layer and the presence of a certain number of water molecules in the second layer, indicating a great influence of the solvent on the reactivity.

Silica-Supported Catalyst for Enantioselective Arylation of Aldehydes under Batch and Continuous-Flow Conditions.

Science.gov (United States)

Watanabe, Satoshi; Nakaya, Naoyuki; Akai, Junichiro; Kanaori, Kenji; Harada, Toshiro

2018-05-04

A silica-supported 3-aryl H 8 -BINOL-derived titanium catalyst exhibited high performance in the enantioselective arylation of aromatic aldehydes using Grignard and organolithium reagents not only under batch conditions but also under continuous-flow conditions. Even with a simple pipet reactor packed with the heterogeneous catalyst, the enantioselective production of chiral diarylmethanols could be achieved through a continuous introduction of aldehydes and mixed titanium reagents generated from the organometallic precursors. The pipet reactor could be used repeatedly in different reactions without appreciable deterioration of the activity.

Enantioselective synthesis of alpha,beta-disubstituted-beta-amino acids.

Science.gov (United States)

Sibi, Mukund P; Prabagaran, Narayanasamy; Ghorpade, Sandeep G; Jasperse, Craig P

2003-10-01

Highly diastereoselective and enantioselective addition of N-benzylhydroxylamine to imides 17 and 20-30 produces alpha,beta-trans-disubstituted N-benzylisoxazolidinones 19 and 31-41. These reactions proceed in 60-96% ee with 93-99% de's using 5 mol % of Mg(NTf2)2 and ligand 18. The product isoxazolidinones can be hydrogenolyzed directly to provide alpha,beta-disubstituted-beta-amino acids.

DNA-based asymmetric catalysis : Sequence-dependent rate acceleration and enantioselectivity

NARCIS (Netherlands)

Boersma, Arnold J.; Klijn, Jaap E.; Feringa, Ben L.; Roelfes, Gerard

2008-01-01

This study shows that the role of DNA in the DNA-based enantioselective Diels-Alder reaction of azachalcone with cyclopentadiene is not limited to that of a chiral scaffold. DNA in combination with the copper complex of 4,4'-dimethyl-2,2'-bipyridine (Cu-L1) gives rise to a rate acceleration of up to

Free radical reactions of monochloramine and hydroxylamine in aqueous solution

International Nuclear Information System (INIS)

Johnson, H.D.; Cooper, William J.; Mezyk, Stephen P.; Bartels, David M.

2002-01-01

The use of Advanced Oxidation Technologies to destroy organic contaminants in drinking water may be impacted by the presence of disinfection chemicals such as monochloramine (NH 2 Cl). To allow a quantitative evaluation of the effect of NH 2 Cl on the destruction of organics in water rate constants for its reaction with the hydrated electron, the hydroxyl radical and the hydrogen atom were determined in this study. The corresponding values of (2.2±0.2)x10 10 , (2.8±0.2)x10 9 , and (1.2±0.1)x10 9 M -1 s -1 , respectively, were incorporated into a kinetic computer model whose predictions were in good agreement with experimental chloramine removal under large scale, steady-state electron-beam irradiation conditions. Rate constants were also determined for the reaction of the hydroxyl radical and hydrogen atom with the chloramine hydration product hydroxylamine to supplement established literature data. Hydroxyl radical rate constants for the basic (NH 2 OH) and acidic (NH 3 OH + ) forms were determined as (8.5±0.4)x10 9 and ≤5x10 7 M -1 s -1 , respectively, while for hydrogen atom reaction, corresponding rate constants of (4.5±0.1)x10 7 and (3.6±1.5)x10 5 M -1 s -1 were found

Modelling substrate specificity and enantioselectivity for lipases and esterases by substrate-imprinted docking

Directory of Open Access Journals (Sweden)

Tyagi Sadhna

2009-06-01

Full Text Available Abstract Background Previously, ways to adapt docking programs that were developed for modelling inhibitor-receptor interaction have been explored. Two main issues were discussed. First, when trying to model catalysis a reaction intermediate of the substrate is expected to provide more valid information than the ground state of the substrate. Second, the incorporation of protein flexibility is essential for reliable predictions. Results Here we present a predictive and robust method to model substrate specificity and enantioselectivity of lipases and esterases that uses reaction intermediates and incorporates protein flexibility. Substrate-imprinted docking starts with covalent docking of reaction intermediates, followed by geometry optimisation of the resulting enzyme-substrate complex. After a second round of docking the same substrate into the geometry-optimised structures, productive poses are identified by geometric filter criteria and ranked by their docking scores. Substrate-imprinted docking was applied in order to model (i enantioselectivity of Candida antarctica lipase B and a W104A mutant, (ii enantioselectivity and substrate specificity of Candida rugosa lipase and Burkholderia cepacia lipase, and (iii substrate specificity of an acetyl- and a butyrylcholine esterase toward the substrates acetyl- and butyrylcholine. Conclusion The experimentally observed differences in selectivity and specificity of the enzymes were reproduced with an accuracy of 81%. The method was robust toward small differences in initial structures (different crystallisation conditions or a co-crystallised ligand, although large displacements of catalytic residues often resulted in substrate poses that did not pass the geometric filter criteria.

Asymmetric Synthesis of Optically Active Spirocyclic Indoline Scaffolds through an Enantioselective Reduction of Indoles

KAUST Repository

Borrmann, Ruediger; Knop, Nils; Rueping, Magnus

2016-01-01

An enantioselective synthesis of spirocyclic indoline scaffolds was achieved by applying an asymmetric iridium-catalyzed hydrogenation of 3H-indoles. Low catalyst loadings and mild reaction conditions provide a broad range of differently substituted

Isolation and Characterization of the 2,2'-Azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) Radical Cation-Scavenging Reaction Products of Arbutin.

Science.gov (United States)

Tai, Akihiro; Ohno, Asako; Ito, Hideyuki

2016-09-28

Arbutin, a glucoside of hydroquinone, has shown strong 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation-scavenging activity, especially in reaction stoichiometry. This study investigated the reaction mechanism of arbutin against ABTS radical cation that caused high stoichiometry of arbutin in an ABTS radical cation-scavenging assay. HPLC analysis of the reaction mixture of arbutin and ABTS radical cation indicated the existence of two reaction products. The two reaction products were purified and identified to be a covalent adduct of arbutin with an ABTS degradation fragment and 3-ethyl-6-sulfonate benzothiazolone. A time-course study of the radical-scavenging reactions of arbutin and the two reaction products suggested that one molecule of arbutin scavenges three ABTS radical cation molecules to generate an arbutin-ABTS fragment adduct as a final reaction product. The results suggest that one molecule of arbutin reduced two ABTS radical cation molecules to ABTS and then cleaved the third ABTS radical cation molecule to generate two products, an arbutin-ABTS fragment adduct and 3-ethyl-6-sulfonate benzothiazolone.

Enantioselective N-Heterocyclic Carbene Catalysis via the Dienyl Acyl Azolium.

Science.gov (United States)

Gillard, Rachel M; Fernando, Jared E M; Lupton, David W

2018-04-16

Herein we report the enantioselective N-heterocyclic carbene catalyzed (4+2) annulation of the dienyl acyl azolium with enolates. The reaction exploits readily accessible acyl fluorides and TMS enol ethers to give a range of highly enantio- and diastereo-enriched cyclohexenes (most >97:3 er and >20:1 dr). The reaction was found to require high nucleophilicity NHC catalysts with mechanistic studies supporting a stepwise 1,6-addition/β-lactonization. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Rate Constant of the Reaction between CH3O2 Radicals and OH Radicals Revisited.

Science.gov (United States)

Assaf, Emmanuel; Song, Bo; Tomas, Alexandre; Schoemaecker, Coralie; Fittschen, Christa

2016-11-17

The reaction between CH 3 O 2 and OH radicals has been studied in a laser photolysis cell using the reaction of F atoms with CH 4 and H 2 O for the simultaneous generation of both radicals, with F atoms generated through 248 nm photolysis of XeF 2 . An experimental setup combining cw-Cavity Ring Down Spectroscopy (cw-CRDS) and high repetition rate laser-induced fluorescence (LIF) to a laser photolysis cell has been used. The absolute concentration of CH 3 O 2 was measured by cw-CRDS, while the relative concentration of OH(v = 0) radicals was determined by LIF. To remove dubiety from the quantification of CH 3 O 2 by cw-CRDS in the near-infrared, its absorption cross section has been determined at 7489.16 cm -1 using two different methods. A rate constant of k 1 = (1.60 ± 0.4) × 10 -10 cm 3 s -1 has been determined at 295 K, nearly a factor of 2 lower than an earlier determination from our group ((2.8 ± 1.4) × 10 -10 cm 3 s -1 ) using CH 3 I photolysis as a precursor. Quenching of electronically excited I atoms (from CH 3 I photolysis) in collision with OH(v = 0) is suspected to be responsible for a bias in the earlier, fast rate constant.

Enantioselectivity in environmental risk assessment of modern chiral pesticides

International Nuclear Information System (INIS)

Ye Jing; Zhao Meirong; Liu Jing; Liu Weiping

2010-01-01

Chiral pesticides comprise a new and important class of environmental pollutants nowadays. With the development of industry, more and more chiral pesticides will be introduced into the market. But their enantioselective ecotoxicology is not clear. Currently used synthetic pyrethroids, organophosphates, acylanilides, phenoxypropanoic acids and imidazolinones often behave enantioselectively in agriculture use and they always pose unpredictable enantioselective ecological risks on non-target organisms or human. It is necessary to explore the enantioselective toxicology and ecological fate of these chiral pesticides in environmental risk assessment. The enantioselective toxicology and the fate of these currently widely used pesticides have been discussed in this review article. - Chiral pesticides could pose unpredictable enantioselective toxicity on non-target organisms.

Enantioselective [3+3] atroposelective annulation catalyzed by N-heterocyclic carbenes

KAUST Repository

Zhao, Changgui

2018-02-05

Axially chiral molecules are among the most valuable substrates in organic synthesis. They are typically used as chiral ligands or catalysts in asymmetric reactions. Recent progress for the construction of these chiral molecules is mainly focused on the transition-metal-catalyzed transformations. Here, we report the enantioselective NHC-catalyzed (NHC: N-heterocyclic carbenes) atroposelective annulation of cyclic 1,3-diones with ynals. In the presence of NHC precatalyst, base, Lewis acid and oxidant, a catalytic C–C bond formation occurs, providing axially chiral α-pyrone−aryls in moderate to good yields and with high enantioselectivities. Control experiments indicated that alkynyl acyl azoliums, acting as active intermediates, are employed to atroposelectively assemble chiral biaryls and such a methodology may be creatively applied to other useful NHC-catalyzed asymmetric transformations.

Reactions of the radical cations of aliphatic aldehydes in freon matrices

International Nuclear Information System (INIS)

Belevskij, V.N.; Belopushkin, S.I.; Feldman, V.I.

1985-01-01

ESR spectra of γ-irradiated solutions of acetic and propionic aldehydes in freon-11 and freon-113 affected by aldehyde concentration, temperature, and the action of light were studied. It is shown that the radical cations are converted into neutral radicals, and the cations CHsub(3)CHsub(2)CHOsup(+). are converted to RCO and CHsub(3)CHCHO due to ion-molecular reactions of proton transfer of hydrogen atom transfer. (author)

EPR and NMR detection of transient radicals and reaction products

International Nuclear Information System (INIS)

Trifunac, A.D.

1981-01-01

Magnetic resonance methods in radiation chemistry are illustrated. The most recent developments in pulsed EPR and NMR studies in pulse radiolysis are outlined with emphasis on the study of transient radicals and their reaction products. 12 figures

Mechanism and kinetics in reactions of caffeic acid with radicals by pulse radiolysis and calculation

International Nuclear Information System (INIS)

Li, Xifeng; Cai, Zhongli; Katsumura, Yosuke

2000-01-01

The interaction of caffeic acid with e aq - , (CH 3 ) 2 (OH) CCH 2 · , CO 2 ·- , H · , ·OH and N 3 · radicals were studied by γ-, pulse radiolysis and molecular orbital calculation. UV-visible spectra of electron/·OH adducts, semi-quinone radicals of caffeic ions, and the stable products from the reactions were derived. The rate constants were determined. The attacked sites and the most favorable structures of the transient radicals were predicted. Reaction mechanisms were proposed. (author)

On the time behaviour of the concentration of pyrazinium radical cations in the early stage of the Maillard reaction

Science.gov (United States)

Stoesser, Reinhard; Klein, Jeannette; Peschke, Simone; Zehl, Andrea; Cämmerer, Bettina; Kroh, Lothar W.

2007-08-01

During the early stage of the Maillard reaction pyrazinium radical cations were detected by ESR within the reaction system D-glucose/glycine. The spectra were characterized by completely resolved hyperfine structure. The partial pressure of oxygen and the radical concentrations were measured directly in the reaction mixture by ESR using solutions of the spin probe TEMPOL and of DPPH, respectively. There are quantitative and qualitative relations of the actual concentration of the radical ions to the partial pressure of oxygen, the temperature-time regime and the mechanical mixing of the reaction system. These macroscopic parameters significantly affect both the induction period and the velocity of the time-dependent formation of free radicals. From in situ variations of p(O 2) and p(Ar) including the connected mixing effects caused by the passing the gases through the reaction mixture, steric and chemical effects of the stabilization of the radical ions were established. The determination of suitable and relevant conditions for stabilization and subsequent radical reactions contributes to the elucidation of the macroscopically known antioxidant activity of Maillard products.

Influence of the medium on the reaction rate of the t-butoxyl radical with iron(II)

International Nuclear Information System (INIS)

Mihaljevic, B.; Razem, D.

2002-01-01

Complete text of publication follows. Tert-butoxyl radicals (t-BuO.) were generated by homolytic photodecomposition of di-tert-butylperoxide using ruby laser flashes at 347 nm. The reaction of t-BuO. radicals with Fe 2+ was studied under pseudo-first order conditions. The quantum yield Φ(Fe 3+ ) was determined by measuring the absorbance of Fe 3+ ion as [FeCl] 2+ complex at 360 nm 2 μs after the flash. According to the equation derived from the reaction scheme, the rate constant k 3 was obtained from the relative rate constant k r (k r =k 0 /k 3 ) and the value of k 0 ; the latter refers to the overall rate of the competing disappearance of t-BuO. radicals from the system (reaction 2), including the highest contribution of β-cleavage. The rate constant k 0 was determined using diphenylmethanol instead of Fe 2+ . The known rate constant of the reaction of t-BuO. radical with diphenylmethanol giving ketyl radicals (6.9 x 10 6 dm 3 mol -1 s -1 ) was applied. The quantum yield of ketyl radicals was determined by measuring the maximum of absorbance at 535 nm. At acid concentration of 0.023 mol dm -3 HCl the rate constant k 3 = 3.4 x 10 8 dm 3 mol -1 s -1 was determined. The relative rate constant increased with an increase of the hydrochloric acid concentration which has been ascribed to the lower stability of t-BuO. radical at higher acidity of the medium. The effect of polarity of the medium on the reaction rate was also determined. Decreasing k 3 in media of increasing polarity were explained by increasing of the β-scission rate of t-BuO. radical with increasing polarity of the medium

Enantioselective Direct α-Amination of Aldehydes via a Photoredox Mechanism: A Strategy for Asymmetric Amine Fragment Coupling

OpenAIRE

Cecere, Giuseppe; Koenig, Christian M.; Alleva, Jennifer L.; MacMillan, David W. C.

2013-01-01

The direct, asymmetric α-amination of aldehydes has been accomplished via a combination of photoredox and organocatalysis. Photon-generated, nitrogen-centered radicals undergo enantioselective α-addition to catalytically formed chiral enamines to directly produce stable α-amino aldehyde adducts bearing synthetically useful amine substitution patterns. Incorporation of a photolabile group on the amine precursor obviates the need to employ a photoredox catalyst in this transformation. Important...

New era of silicon technologies due to radical reaction based semiconductor manufacturing

International Nuclear Information System (INIS)

Ohmi, Tadahiro; Hirayama, Masaki; Teramoto, Akinobu

2006-01-01

Current semiconductor technology, the so-called the molecule reaction based semiconductor manufacturing, now faces a very severe standstill due to the drastic increase of gate leakage currents and drain leakage currents. Radical reaction based semiconductor manufacturing has been developed to completely overcome the current standstill by introducing microwave excited high density plasma with very low electron temperatures and without accompanying charge-up damage. The introduction of radical reaction based semiconductor manufacturing has made it possible to fabricate LSI devices on any crystal orientation Si substrate surface as well as (100) Si substrate surfaces, and to eliminate a very severe limitation to the antenna ratio in the circuit layout patterns, which is strictly limited to less than 100-200 in order to obtain a relatively high production yield. (topical review)

Optimisation of the enantioselective biocatalytic hydrolysis of naproxen ethyl ester using ChiroCLEC-CR

CSIR Research Space (South Africa)

Brady, D

2004-03-04

Full Text Available In a biocatalytic reaction the immobilized lipase ChiroCLEC-CR enantioselectively hydrolysed a naproxen ethyl ester racemate, yielding (S)-naproxen with an enantiomeric excess of more than 98%, an enantiomeric ratio (E) of more than 100...

Enhancing the potential of enantioselective organocatalysis with light

Science.gov (United States)

Silvi, Mattia; Melchiorre, Paolo

2018-02-01

Organocatalysis—catalysis mediated by small chiral organic molecules—is a powerful technology for enantioselective synthesis, and has extensive applications in traditional ionic, two-electron-pair reactivity domains. Recently, organocatalysis has been successfully combined with photochemical reactivity to unlock previously inaccessible reaction pathways, thereby creating new synthetic opportunities. Here we describe the historical context, scientific reasoning and landmark discoveries that were essential in expanding the functions of organocatalysis to include one-electron-mediated chemistry and excited-state reactivity.

Highly Functionalized Cyclopentane Derivatives by Tandem Michael Addition/Radical Cyclization/Oxygenation Reactions

Czech Academy of Sciences Publication Activity Database

Holan, Martin; Pohl, Radek; Císařová, I.; Klepetářová, Blanka; Jones, P. G.; Jahn, Ullrich

2015-01-01

Roč. 21, č. 27 (2015), s. 9877-9888 ISSN 0947-6539 R&D Projects: GA ČR GA13-40188S Institutional support: RVO:61388963 Keywords : cyclization * domino reactions * electron transfer * Michael addition * radical reactions Subject RIV: CC - Organic Chemistry Impact factor: 5.771, year: 2015

Reaction of hydroxyl radicals with ammonia in liquid water at elevated temperatures

DEFF Research Database (Denmark)

Hickel, B.; Sehested, K.

1992-01-01

The reaction of hydroxyl radical with ammonia in aqueous solutions has been studied by pulse radiolysis in the temperature range 20-200-degrees-C. The rate constant of the reaction was determined by monitoring the decay of the OH radical absorption at 260 nm for different concentrations of ammonia....... At room temperature the rate constant is (9.7 +/- 1) x 10(7) dm3 mol-1 s-1. In the whole range of temperatures the Tate constant follows Arrhenius law with an activation energy of (5.7 +/- 1) kJ mol-1. The protective effect of dissolved hydrogen on the radiolytic decomposition of ammon a is discussed....

Reactions of radicals with lecithin bilayers

International Nuclear Information System (INIS)

Barber, D.J.W.; Thomas, J.K.

1978-01-01

The kinetics of reaction of .OH and e/sub aq/ - with lecithin bilayers have been measured. The rate for .OH + lecithin is 5.1 +- 0.9 x 10 8 M -1 sec -1 while the e/sub aq/ - + lecithin rate is very slow. When a solute such as pyrene is solubilized in the bilayer, .OH and e/sub aq/ - may react with the solute; rates of 1.65 +- 0.12 x 10 9 M -1 sec -1 and 7 x 10 7 M -1 sec -1 have been measured for reaction of .OH and e/sub aq/ - , respectively, with pyrene in lecithin. These rates are lower than those observed for similar reactions in homogeneous systems. This is explained in terms of (a) the protective effect of the bilayer, this being especially true for e/sub aq/ - which does not readily leave the aqueous phase, and (b) in terms of the restricted diffusion imposed on the reactive species by the bilayer. The kinetics in these model systems are relevant to reactions of radicals with membranes. Long-term alteration in the model membrane following .OH attack is manifested in terms of damage to the head group, increasing water penetration of the bilayer, and of cross-linking with the membrane, thereby restricting motion in the interior of the bilayer. Increased rigidity and leakiness of membranes is an expected consequence of radiation damage

Highly Enantioselective Construction of Tertiary Thioethers and Alcohols via Phosphine-Catalyzed Asymmetric γ-Addition reactions of 5H-Thiazol-4-ones and 5H-Oxazol-4-ones: Scope and Mechanistic Understandings

KAUST Repository

Wang, Tianli

2015-06-02

Phosphine-catalyzed highly enantioselective γ-additions of 5H-thiazol-4-ones and 5H-oxazol-4-ones to allenoates have been developed for the first time. With the employment of amino-acid derived bifunctional phosphines, a wide range of substituted 5H-thiazol-4-one and 5H-oxazol-4-one derivatives bearing heteroarom (S or O)-containing tertiary chiral centers were constructed in high yields and excellent enantioselectivities. The reported method provides a facile access to enantioenriched tertiary thioether/alcohols. The mechanism of γ-addition reaction was investigated by performing DFT calculations, and the hydrogen bonding interactions between the Brønsted acid moiety of the phosphine catalysts and the “C=O” unit of donor molecules were shown to be crucial in asymmetric induction.

Highly Enantioselective Construction of Tertiary Thioethers and Alcohols via Phosphine-Catalyzed Asymmetric γ-Addition reactions of 5H-Thiazol-4-ones and 5H-Oxazol-4-ones: Scope and Mechanistic Understandings

KAUST Repository

Wang, Tianli; Yu, Zhaoyuan; Hoon, Ding Long; Huang, Kuo-Wei; Lan, Yu; Lu, Yixin

2015-01-01

Phosphine-catalyzed highly enantioselective γ-additions of 5H-thiazol-4-ones and 5H-oxazol-4-ones to allenoates have been developed for the first time. With the employment of amino-acid derived bifunctional phosphines, a wide range of substituted 5H-thiazol-4-one and 5H-oxazol-4-one derivatives bearing heteroarom (S or O)-containing tertiary chiral centers were constructed in high yields and excellent enantioselectivities. The reported method provides a facile access to enantioenriched tertiary thioether/alcohols. The mechanism of γ-addition reaction was investigated by performing DFT calculations, and the hydrogen bonding interactions between the Brønsted acid moiety of the phosphine catalysts and the “C=O” unit of donor molecules were shown to be crucial in asymmetric induction.

Reaction of single-standard DNA with hydroxyl radical generated by iron(II)-ethylenediaminetetraacetic acid

International Nuclear Information System (INIS)

Prigodich, R.V.; Martin, C.T.

1990-01-01

This study demonstrates that the reaction of Fe(II)-EDTA and hydrogen peroxide with the single-stranded nucleic acids d(pT) 70 and a 29-base sequence containing a mixture of bases results in substantial damage which is not directly detected by gel electrophoresis. Cleavage of the DNA sugar backbone is enhanced significantly after the samples are incubated at 90 degree C in the presence of piperidine. The latter reaction is used in traditional Maxam-Gilbert DNA sequencing to detect base damage, and the current results are consistent with reaction of the hydroxyl radical with the bases in single-stranded DNA (although reaction with sugar may also produce adducts that are uncleaved but labile to cleavage by piperidine). We the authors propose that hydroxyl radicals may react preferentially with the nucleic acid bases in ssDNA and that reaction of the sugars in dsDNA is dominant because the bases are sequestered within the double helix. These results have implications both for the study of single-stranded DNA binding protein binding sites and for the interpretation of experiments using the hydroxyl radical to probe DNA structure or to footprint double-stranded DNA binding protein binding sites

Rate constant for reaction of vitamin C with protein radicals in γ-irradiated aqueous albumin solution at 295K

International Nuclear Information System (INIS)

Miyazaki, Tetsuo; Yoshimura, Toru; Mita, Kazuya; Suzuki, Keiji; Watanabe, Masami

1995-01-01

When an aqueous solution of albumin (0.1 kg dm -3 ) is irradiated by γ-rays at 295 K, albumin radicals with a long lifetime are observed by ESR. The reaction of vitamin C with the albumin radicals has been studied at 295 K in the albumin solution, which is considered as a model of cells. The rate constant for the reaction of vitamin C with the albumin radicals was measured as 0.014 dm 3 mol -1 S -1 , which is much smaller than the reported constants (10 6 -10 10 dm 3 mol -1 s -1 ) for the reaction of vitamin C with radicals in a dilute aqueous solution. The small rate constant for the reaction of vitamin C is ascribed to the reaction in polymer coils in the albumin solution, since vitamin C and albumin radicals diffuse very slowly in the coils. (author)

Selected specific rates of reactions of transients from water in aqueous solution. III. Hydroxyl radical and perhydroxyl radical and their radical ions

Energy Technology Data Exchange (ETDEWEB)

Ross, F; Ross, A B

1977-01-01

Rates of reactions of OH and HO/sub 2/ with organic and inorganic molecules, ions and transients in aqueous solution have been tabulated, as well as the rates for the corresponding radical ions in aqueous solution (O/sup -/ and O/sub 2//sup -/). Most of the rates have been obtained by radiation chemistry methods, both pulsed and steady-state; data from photochemistry and thermal methods are also included. Rates for over one thousand reactions are listed.

Preparative and mechanistic studies toward the rational development of catalytic, enantioselective selenoetherification reactions.

Science.gov (United States)

Denmark, Scott E; Kalyani, Dipannita; Collins, William R

2010-11-10

A systematic investigation into the Lewis base catalyzed, asymmetric, intramolecular selenoetherification of olefins is described. A critical challenge for the development of this process was the identification and suppression of racemization pathways available to arylseleniranium ion intermediates. This report details a thorough study of the influences of the steric and electronic modulation of the arylselenenyl group on the configurational stability of enantioenriched seleniranium ions. These studies show that the 2-nitrophenyl group attached to the selenium atom significantly attenuates the racemization of seleniranium ions. A variety of achiral Lewis bases catalyze the intramolecular selenoetherification of alkenes using N-(2-nitrophenylselenenyl)succinimide as the electrophile along with a Brønsted acid. Preliminary mechanistic studies suggest the intermediacy of ionic Lewis base-selenium(II) adducts. Most importantly, a broad survey of chiral Lewis bases revealed that 1,1'-binaphthalene-2,2'-diamine (BINAM)-derived thiophosphoramides catalyze the cyclization of unsaturated alcohols in the presence of N-(2-nitrophenylselenenyl)succinimide and methanesulfonic acid. A variety of cyclic seleno ethers were produced in good chemical yields and in moderate to good enantioselectivities, which constitutes the first catalytic, enantioselective selenofunctionalization of unactivated olefins.

Radical Abstraction Reactions with Concerted Fragmentation in the Chain Decay of Nitroalkanes

Science.gov (United States)

Denisov, E. T.; Shestakov, A. F.

2018-05-01

Reactions of the type X• + HCR2CH2NO2 → XH + R2C=CH2 + N•O2 are exothermic, due to the breaking of weak C-N bonds and the formation of energy-intensive C=C bonds. Quantum chemistry calculations of the transition state using the reactions of Et• and EtO• with 2-nitrobutane shows that such reactions can be categorized as one-step, due to the extreme instability of the intermediate nitrobutyl radical toward decay with the formation of N•O2. Kinetic parameters that allow us to calculate the energy of activation and rate constant of such a reaction from its enthalpy are estimated using a model of intersecting parabolas. Enthalpies, energies of activation, and rate constants are calculated for a series of reactions with the participation of Et•, EtO•, RO•2, N•O2 radicals on the one hand and a series of nitroalkanes on the other. A new kinetic scheme of the chain decay of nitroalkanes with the participation of abstraction reactions with concerted fragmentation is proposed on the basis of the obtained data.

Determination of the number of radicals in the initial chain reactions by mathematical methods

Directory of Open Access Journals (Sweden)

Pejović Branko B.

2009-01-01

Full Text Available Starting from the fact that the real mechanism in a chemical equation takes places through a certain number of radicals which participate in simultaneous reactions and initiate chain reactions according to a particular pattern, the aim of this study is to determine their number in the first couple of steps of the reaction. Based on this, the numbers of radicals were determined in the general case, in the form of linear difference equations, which, by certain mathematical transformations, were reduced to one equation that satisfies a particular numeric series, entirely defined if its first members are known. The equation obtained was solved by a common method developed in the theory of numeric series, in which its solutions represent the number of radicals in an arbitrary step of the reaction observed, in the analytical form. In the final part of the study, the method was tested and verified using two characteristic examples from general chemistry. The study also gives a suggestion of a more efficient procedure by reducing the difference equation to a lower order.

Mechanism and kinetics in reactions of caffeic acid with radicals by pulse radiolysis and calculation

Energy Technology Data Exchange (ETDEWEB)

Li, Xifeng; Cai, Zhongli; Katsumura, Yosuke [Tokyo Univ., Tokai, Ibaraki (Japan). Nuclear Engineering Research Lab

2000-03-01

The interaction of caffeic acid with e{sub aq}{sup -}, (CH{sub 3}){sub 2}(OH) CCH{sub 2}{sup {center_dot}}, CO{sub 2}{sup {center_dot}}{sup -}, H{sup {center_dot}}, {center_dot}OH and N{sub 3}{sup {center_dot}} radicals were studied by {gamma}-, pulse radiolysis and molecular orbital calculation. UV-visible spectra of electron/{center_dot}OH adducts, semi-quinone radicals of caffeic ions, and the stable products from the reactions were derived. The rate constants were determined. The attacked sites and the most favorable structures of the transient radicals were predicted. Reaction mechanisms were proposed. (author)

The enantioselective total synthesis of (+)-clusianone.

Science.gov (United States)

Horeischi, Fiene; Guttroff, Claudia; Plietker, Bernd

2015-02-11

(+)-Clusianone, an exo-type B PPAP with reported anti-HIV and chemoprotective activities, was synthesized in eleven steps with 97% ee starting from acetylacetone. An enantioselective decarboxylative Tsuji-Trost-allylation and a Ru-catalyzed ring-closing metathesis-decarboxylative allylation were used to control both diastereo- and enantioselectivity.

Enantioselective pharmacokinetics of sibutramine in rat.

Science.gov (United States)

Noh, Keumhan; Bae, Kyoungjin; Min, Bokyoung; Kim, Eunyoung; Kwon, Kwang-il; Jeong, Taecheon; Kang, Wonku

2010-02-01

Racemic sibutramine is widely used to treat obesity owing to its inhibition of serotonin and noradrenaline reuptake in synapses. Although the enantioselective effects of sibutramine and its two active desmethyl-metabolites, monodesmethylsibutramine (MDS) and didesmethylsibutramine (DDS), on anorexia and energy expenditure have been elucidated, the enantioselective pharmacokinetics of sibutramine are still unclear. Therefore, we aimed to characterize the enantioselective pharmacokinetics of sibutramine and its metabolites in plasma and urine following an intravenous and a single oral administration of sibutramine in rats. The absolute bioavailability of sibutramine was only about 7%. The pharmacologically less effective S-isomer of DDS was predominant in the plasma: the C ( max ) and the AUC ( inf ) were 28 and 30 times higher than those of the R-isomer, respectively (psibutramine metabolites MDS and DDS were present at lower concentrations, owing to their rapid biotransformation to hydroxylated and/or carbamoylglucuronized forms and their faster excretion in the urine. The present study is the first to elucidate the enantioselective pharmacokinetics of sibutramine in rats.

Reaction between aminoalkyl radicals and akyl halides: Dehalogenation by electron transfer?

Science.gov (United States)

Lalevée, J.; Fouassier, J. P.; Blanchard, N.; Ingold, K. U.

2011-07-01

Aminoalkyl radicals, such as Et2NCrad HCH3, have low oxidation potentials and are therefore powerful reducing agents. We have found that Et2NCrad HCH3 reacts with CCl4 and CBr4 in di-tert-butyl peroxide with bimolecular rate constants (measured by LFP) close, or equal, to the diffusion-controlled limit. For the less reactive halide, CH2Br2, the reaction rate is increased substantially by the addition of acetonitrile as a co-solvent. It is tentatively concluded that these reactions occur by electron-transfer from the aminoalkyl to the organohalide with formation of the iminium ion, Et2N+dbnd CHCH3 (NMR detection), halide ion and a halomethyl radical, e.g., rad CCl3 and rad CHCl2 (ESR, spin-trapping detection).

Diastereoselective and enantioselective reduction of tetralin-1,4-dione

Directory of Open Access Journals (Sweden)

2008-10-01

Full Text Available BackgroundThe chemistry of tetralin-1,4-dione, the stable tautomer of 1,4-dihydroxynaphthalene, has not been explored previously. It is readily accessible and offers interesting opportunities for synthesis.ResultsThe title reactions were explored. L-Selectride reduced the diketone to give preferentially the cis-diol (d.r. 84 : 16. Red-Al gave preferentially the trans-diol (d.r. 13 : 87. NaBH4, LiAlH4, and BH3 gave lower diastereoselectivities (yields: 76–98%. Fractional crystallization allowed isolation of the cis-diol and the trans-diol (55% and 66% yield, respectively. Borane was used to cleanly give the mono-reduction product. Highly enantioselective CBS reductions afforded the trans-diol (72% yield, 99% ee and the mono-reduction product (81%, 95% ee.ConclusionDiastereoselective and enantioselective reductions of the unexplored tetralin-1,4-dione provides a very convenient entry into a number of synthetically highly attractive 1,4-tetralindiols and 4-hydroxy-1-tetralone.

Diastereoselective and enantioselective reduction of tetralin-1,4-dione.

Science.gov (United States)

Kündig, E Peter; Enriquez-Garcia, Alvaro

2008-01-01

The chemistry of tetralin-1,4-dione, the stable tautomer of 1,4-dihydroxynaphthalene, has not been explored previously. It is readily accessible and offers interesting opportunities for synthesis. The title reactions were explored. L-Selectride reduced the diketone to give preferentially the cis-diol (d.r. 84 : 16). Red-Al gave preferentially the trans-diol (d.r. 13 : 87). NaBH(4), LiAlH(4), and BH(3) gave lower diastereoselectivities (yields: 76-98%). Fractional crystallization allowed isolation of the cis-diol and the trans-diol (55% and 66% yield, respectively). Borane was used to cleanly give the mono-reduction product. Highly enantioselective CBS reductions afforded the trans-diol (72% yield, 99% ee) and the mono-reduction product (81%, 95% ee). Diastereoselective and enantioselective reductions of the unexplored tetralin-1,4-dione provides a very convenient entry into a number of synthetically highly attractive 1,4-tetralindiols and 4-hydroxy-1-tetralone.

Enantioselective small molecule synthesis by carbon dioxide fixation using a dual Brønsted acid/base organocatalyst.

Science.gov (United States)

Vara, Brandon A; Struble, Thomas J; Wang, Weiwei; Dobish, Mark C; Johnston, Jeffrey N

2015-06-17

Carbon dioxide exhibits many of the qualities of an ideal reagent: it is nontoxic, plentiful, and inexpensive. Unlike other gaseous reagents, however, it has found limited use in enantioselective synthesis. Moreover, unprecedented is a tool that merges one of the simplest biological approaches to catalysis-Brønsted acid/base activation-with this abundant reagent. We describe a metal-free small molecule catalyst that achieves the three component reaction between a homoallylic alcohol, carbon dioxide, and an electrophilic source of iodine. Cyclic carbonates are formed enantioselectively.

Solution phase and membrane immobilized iron-based free radical reactions: Fundamentals and applications for water treatment

Science.gov (United States)

Lewis, Scott Romak

Membrane-based separation processes have been used extensively for drinking water purification, wastewater treatment, and numerous other applications. Reactive membranes synthesized through functionalization of the membrane pores offer enhanced reactivity due to increased surface area at the polymer-solution interface and low diffusion limitations. Oxidative techniques utilizing free radicals have proven effective for both the destruction of toxic organics and non-environmental applications. Most previous work focuses on reactions in the homogeneous phase; however, the immobilization of reactants in membrane pores offers several advantages. The use of polyanions immobilized in a membrane or chelates in solution prevents ferric hydroxide precipitation at near-neutral pH, a common limitation of iron(Fe(II/III))-catalyzed hydrogen peroxide (H 2O2) decomposition. The objectives of this research are to develop a membrane-based platform for the generation of free radicals, degrade toxic organic compounds using this and similar solution-based reactions, degrade toxic organic compounds in droplet form, quantify hydroxyl radical production in these reactions, and develop kinetic models for both processes. In this study, a functionalized membrane containing poly(acrylic acid) (PAA) was used to immobilize iron ions and conduct free radical reactions by permeating H2O2 through the membrane. The membrane's responsive behavior to pH and divalent cations was investigated and modeled. The conversion of Fe(II) to Fe(III) in the membrane and its effect on the decomposition of hydrogen peroxide were monitored and used to develop kinetic models for predicting H2O2 decomposition in these systems. The rate of hydroxyl radical production, and hence contaminant degradation can be varied by changing the residence time, H2O2 concentration, and/or iron loading. Using these membrane-immobilized systems, successful removal of toxic organic compounds, such as pentachlorophenol (PCP), from water

Combined experimental and theoretical study of the mechanism and enantioselectivity of palladium-catalyzed intermolecular Heck coupling

DEFF Research Database (Denmark)

Henriksen, Signe Teuber; Norrby, Per-Ola; Kaukoranta, Päivi

2008-01-01

. The steric interactions in this transition state fully account for the enantioselectivity observed with the ligands studied. The calculations also predict relative reactivity and nonlinear mixing effects for the investigated ligands; these predictions are fully validated by experimental testing. Finally......The asymmetric Heck reaction using P,N-ligands has been studied by a combination of theoretical and experimental methods. The reaction follows Halpern-style selectivity; that is, the major isomer is produced from the least favored form of the pre-insertion intermediate. The initially formed Ph......, the low conversion observed with some catalysts was found to be caused by inactivation due to weak binding of the ligand to Pd(0). Adding monodentate PPh3 alleviated the precipitation problem without deteriorating the enantioselectivity and led to one of the most effective catalytic systems to date....

Reaction of long-lived radicals and vitamin C in γ-irradiated mammalian cells and their model system at 295 K. Tunneling reaction in biological system

International Nuclear Information System (INIS)

Matsumoto, Takuro; Miyazaki, Tetsuo; Kosugi, Yoshio; Kumada, Takayuki; Koyama, Sinji; Kodama, Seiji; Watanabe, Masami.

1996-01-01

When golden hamster embryo (GHE) cells or concentrated albumin solution (0.1 kg dm -3 ) that is a model system of cells is irradiated with γ-rays at 295 K, organic radicals produced can be observed by ESR. The organic radicals survive at both 295 K and 310 K for such a long time as 20 hr. The long-lived radicals in GHE cells and the albumin solution react with vitamin C by the rate constants of 0.007 dm 3 mol -1 s -1 and 0.014 dm 3 mol -1 s -1 , respectively. The long-lived radicals in human cells cause gene mutation, which is suppressed by addition of vitamin C. The isotope effect on the rate constant (k) for the reaction of the long-lived radicals and vitamin C has been studied in the albumin solution by use of protonated vitamin C and deuterated vitamin C. The isotope effect (k H /k D ) was more than 20-50 and was interpreted in terms of tunneling reaction. When GHE cells or the aqueous albumin solution (0.1 kg dm -3 ) is irradiated with γ-rays at 295 K, organic radicals produced survive for more than 24 hr at room temperature. Very recently we have found that vitamin C reacts with the long-lived organic radicals in the γ-irradiated albumin solution at high concentration of 0.1 kg dm -3 by the rate constant of 0.014 dm 3 mol -1 s -1 . Since most of reactions in biological systems including the reaction of vitamin C are a transfer of a hydrogen atom or a proton that has a large wave character, it is generally expected that the tunneling reaction may play an important role in biological systems at room temperature. The studies of isotope effects on reactions will give an information on the contribution of tunneling reaction. (J.P.N.)

Reactions of the nitrate radical with a series og reduced organic sulfur-compounds in air

DEFF Research Database (Denmark)

JENSEN, NR; HJORTH, J; LOHSE, C

1992-01-01

A 480 L evacuable reaction chamber, equipped with FT-IR spectroscopy on-line and ion chromatography off-line, has been used to study the gas phase reaction between the nitrate radical, NO3, and the reduced organic sulphur compounds CH3CH2SH, (CH3CH2)2S, (CH3CH2)2S2, and CH3CH2SCH3 in air. The pro......A 480 L evacuable reaction chamber, equipped with FT-IR spectroscopy on-line and ion chromatography off-line, has been used to study the gas phase reaction between the nitrate radical, NO3, and the reduced organic sulphur compounds CH3CH2SH, (CH3CH2)2S, (CH3CH2)2S2, and CH3CH2SCH3 in air......, and CH3SSCH3 lead to the conclusion that all these species, in the reaction with the NO3 radical, follow a similar degradation mechanism producing SO2, H2SO4, R-SO3H, R-CHO, and R-CH2ONO2, as the main reaction products. The inital step of the reaction of NO3 with R-S-R and R-S- H type (R = CH3, CH2CH3...

One pot 'click' reactions : tandem enantioselective biocatalytic epoxide ring opening and [3+2] azide alkyne cycloaddition

NARCIS (Netherlands)

Campbell-Verduyn, Lachlan S.; Szymanski, Wiktor; Postema, Christiaan P.; Dierckx, Rudi A.; Elsinga, Philip H.; Janssen, Dick B.; Feringa, Ben L.

2010-01-01

Halohydrin dehalogenase (HheC) can perform enantioselective azidolysis of aromatic epoxides to 1,2-azido alcohols which are subsequently ligated to alkynes producing chiral hydroxy triazoles in a one-pot procedure with excellent enantiomeric excess.

Efficient and highly enantioselective formation of the all-carbon quaternary stereocentre of lyngbyatoxin A

DEFF Research Database (Denmark)

Vital, Paulo J.V.; Tanner, David

2006-01-01

Indole 25, an advanced intermediate in a projected enantioselective total synthesis of lyngbyatoxin A 1, was prepared from allylic alcohol 11 in 9 steps and >95% ee, key transformations being the enantiospecific rearrangement of vinyl epoxide 14 and the Hemetsberger-Knittel reaction of azide 24....

Fast reactions of organic anion radicals with organic halides in hexamethylphosphoric triamide studied by pulse radiolysis

International Nuclear Information System (INIS)

Honda, Eiji; Tokuda, Masao; Yoshida, Hiroshi; Ogasawara, Masaaki

1987-01-01

Fast reactions of diethyl fumarate anion radical (DEF - ) and fluorenone anion radical (Fl - ) with various organic halides (RX) in hexamethylphosphoric triamide have been studied by means of ns pulse radiolysis at room temperature. Reactions of acetophenone anion radical were also studied for comparison. It was found that the reaction rate of Fl - was subject to the steric and resonance effects of R groups of RX in accord with the classical concept of S N 2 reactions: the rate constant was reduced by 2 orders of magnitude by the steric effect when R was changed from ethyl to bulky isopropyl or t-butyl, and it was still large by the resonance effect of R even if R was changed from ethyl to an allyl or a benzyl group. While the reaction rate of DEF - was not much affected when R was changed to more bulky groups, the rate constant was correlated to the reduction potential of RX. The results were interpreted in terms of a VB correlation diagram approach or rate-equilibrium relationship within a framework of S N 2 reactions. (author)

Electron spin resonance of spin-trapped radicals of amines and polyamines. Hydroxyl radical reactions in aqueous solutions and. gamma. radiolysis in the solid state

Energy Technology Data Exchange (ETDEWEB)

Mossoba, M.M.; Rosenthal, I.; Riesz, P. (National Cancer Inst., Bethesda, MD (USA))

1982-06-15

The reactions of hydroxyl radicals with methylamine, dimethylamine, trimethylamine, diethylamine, sec-butylamine, ethylene-diamine, 1,3-diaminopropane, putrescine, cadaverine, 1,7-diaminoheptane, ornithine, spermidine, spermine, agmatine, and arcaine in aqueous solutions have been investigated by spin-trapping and esr. Hydroxyl radicals were generated by the uv photolysis of H/sub 2/O/sub 2/ and 2-methyl-2-nitrosopropane (MNP) was used as the spin-trap. The effects of ionizing radiation on the same polyamines in the polycrystalline state were also investigated. The free radicals produced by ..gamma..-radiolysis of these solids at room temperature in the absence of air were identified by dissolution in aqueous solutions of MNP. The predominant reaction of OH radicals with amines and polyamines below pH 7 was the abstraction of hydrogen atoms from a carbon that is not adjacent to the protonated amino group. For agmatine and arcaine which contain guanidinium groups abstraction occurred from the ..cap alpha..-CH. Dimethylamine was oxidized to the dimethylnitroxyl radical by H/sub 2/O/sub 2/ in the dark. ..gamma..-Radiolysis of polyamines in the polycrystalline state generated radicals due to H-abstraction from either the ..cap alpha..-Ch or from a carbon atom in the middle of the alkyl chain. The deamination radical was obtained from ornithine.

Sub-millitesla magnetic field effects on the recombination reaction of flavin and ascorbic acid radicals

Energy Technology Data Exchange (ETDEWEB)

Evans, Emrys W.; Henbest, Kevin B.; Timmel, Christiane R., E-mail: christiane.timmel@chem.ox.ac.uk, E-mail: stuart.mackenzie@chem.ox.ac.uk [Department of Chemistry, Centre for Advanced Electron Spin Resonance, University of Oxford, Oxford (United Kingdom); Kattnig, Daniel R.; Hore, P. J.; Mackenzie, Stuart R., E-mail: christiane.timmel@chem.ox.ac.uk, E-mail: stuart.mackenzie@chem.ox.ac.uk [Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford (United Kingdom)

2016-08-28

Even though the interaction of a <1 mT magnetic field with an electron spin is less than a millionth of the thermal energy at room temperature (k{sub B}T), it still can have a profound effect on the quantum yields of radical pair reactions. We present a study of the effects of sub-millitesla magnetic fields on the photoreaction of flavin mononucleotide with ascorbic acid. Direct control of the reaction pathway is achieved by varying the rate of electron transfer from ascorbic acid to the photo-excited flavin. At pH 7.0, we verify the theoretical prediction that, apart from a sign change, the form of the magnetic field effect is independent of the initial spin configuration of the radical pair. The data agree well with model calculations based on a Green’s function approach that allows multinuclear spin systems to be treated including the diffusive motion of the radicals, their spin-selective recombination reactions, and the effects of the inter-radical exchange interaction. The protonation states of the radicals are uniquely determined from the form of the magnetic field-dependence. At pH 3.0, the effects of two chemically distinct radical pair complexes combine to produce a pronounced response to ∼500 μT magnetic fields. These findings are relevant to the magnetic responses of cryptochromes (flavin-containing proteins proposed as magnetoreceptors in birds) and may aid the evaluation of effects of weak magnetic fields on other biologically relevant electron transfer processes.

Protonation Reaction of Benzonitrile Radical Anion and Absorption of Product

DEFF Research Database (Denmark)

Holcman, Jerzy; Sehested, Knud

1975-01-01

The rate constant for the protonation of benzonitrile radical anions formed in pulse radiolysis of aqueous benzonitrile solutions is (3.5 ± 0.5)× 1010 dm3 mol–1 s–1. A new 270 nm absorption band is attributed to the protonated benzonitrile anion. The pK of the protonation reaction is determined t...

Free Br atom and free radical reactions in the radiolysis of 1,2 dibromoethane (DBE) in air free aqueous solutions

International Nuclear Information System (INIS)

Lal, Manohar

1986-01-01

G(Br - ) have been reported in the free radical degradation of 1,2 DBE in Ar - and N 2 O-saturated solutions. It is clear from the results that a small chain reaction occurs, t-butanol radical reacts with 1,2 DBE to give Br - . At pH 12.3, high (Br - ) are attributed to another chain reaction involving O - radical anion. Dose rate studies confirm the occurrence of chain reaction. (author). 5 refs

Organocatalytic Enantioselective Pictet-Spengler Approach to Biologically Relevant 1-Benzyl-1,2,3,4-Tetrahydroisoquinoline Alkaloids

NARCIS (Netherlands)

Ruiz-Olalla, A.; Würdemann, M.A.; Wanner, M.J.; Ingemann, S.; van Maarseveen, J.H.; Hiemstra, H.

2015-01-01

A general procedure for the synthesis of 1-benzyl-1,2,3,4-tetrahydroisoquinolines was developed, based on organocatalytic, regio- and enantioselective Pictet-Spengler reactions (86-92% ee) of N-(o-nitrophenylsulfenyl)-2-arylethyl-amines with arylacetaldehydes. The presence of the

An efficient catalyst for asymmetric Reformatsky reaction

Indian Academy of Sciences (India)

rate enantioselectivity using N,N-dialkylnorephedrines as chiral ligands. ..... temperatures also, there was no product conversion. ... Optimization of reaction conditions for asymmetric Reformatsky reaction between benzaldehyde and α-.

A combined high-temperature experimental and theoretical kinetic study of the reaction of dimethyl carbonate with OH radicals

KAUST Repository

Khaled, Fathi; Giri, Binod; Szőri, Milá n; Mai, Tam V.-T.; Huynh, Lam K.; Farooq, Aamir

2017-01-01

The reaction kinetics of dimethyl carbonate (DMC) and OH radicals were investigated behind reflected shock waves over the temperature range of 872-1295 K and at pressures near 1.5 atm. Reaction progress was monitored by detecting OH radicals at 306.69 nm using a UV laser absorption technique. The rate coefficients for the reaction of DMC with OH radicals were extracted using a detailed kinetic model developed by Glaude et al. (Proc. Combust. Inst. 2005, 30(1), 1111-1118). The experimental rate coefficients can be expressed in Arrhenius form as: kexpt'l = 5.15 × 10(13) exp(-2710.2/T) cm(3) mol(-1) s(-1). To explore the detailed chemistry of the DMC + OH reaction system, theoretical kinetic analyses were performed using high-level ab initio and master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) calculations. Geometry optimization and frequency calculations were carried out at the second-order Møller-Plesset (MP2) perturbation level of theory using Dunning's augmented correlation consistent-polarized valence double-ζ basis set (aug-cc-pVDZ). The energy was extrapolated to the complete basis set using single point calculations performed at the CCSD(T)/cc-pVXZ (where X = D, T) level of theory. For comparison purposes, additional ab initio calculations were also carried out using composite methods such as CBS-QB3, CBS-APNO, G3 and G4. Our calculations revealed that the H-abstraction reaction of DMC by OH radicals proceeds via an addition elimination mechanism in an overall exothermic process, eventually forming dimethyl carbonate radicals and H2O. Theoretical rate coefficients were found to be in excellent agreement with those determined experimentally. Rate coefficients for the DMC + OH reaction were combined with literature rate coefficients of four straight chain methyl ester + OH reactions to extract site-specific rates of H-abstraction from methyl esters by OH radicals.

Investigations of structure, bonding, and reactions of radiation-induced free radicals in the solid state using electron spin resonance spectroscopy

International Nuclear Information System (INIS)

Hudson, R.L.

1978-01-01

Electron spin resonance spectroscopy (ESR) has been used to study the structure, bonding, and reactions of several types of free radicals produced by γ irradiation of solids at 77K. Well-defined spectral patterns and the use of photolysis and annealing treatments assisted the analyses and interpretations. The radical anion BF 3 - was generated and identified unequivocally in a matrix of tetramethylsilane at 77K. Both the ESR data and theoretical calculations support a pyramidal structure with a bond angle of about 110 0 . The present experiments showed that BF 3 - has ESR parameters consistent with those of the isoelectronic radicals CF 3 , NF 3 + , and F 2 NO. γ irradiation of polycrystalline trimethyl borate at 77K gave an ESR spectrum which was assigned to the dimer radical anion [(MeO) 3 B.B(OMe) 3 ] - . Radical anions of dialkyl carbonates were observed for the first time and found to undergo a β-scission reaction to produce alkyl radicals. This free radical reaction is unusual in that it proceeds both thermally and photochemically. For the dimethyl carbonate radical anion, 13 C parameters were obtained from a 13 C enriched sample. The photolysis of trapped radicals in γ irradiated carboxylic esters, RC(O)OR', was studied by ESR spectroscopy and two different reactions were characterized. Two hypervalent silicon radical anions were prepared and examined in SI(OCH 3 ) 4 . The results of the present work thus represent the first complete sets of data on the silicon 3s and 3p spin densities for such species. The first PL 3 - radical anion was prepared by the γ irradiation of crystalline trimethylphosphite, and identified through its photolysis reactions and from the results of radiation chemical experiments

Reactions of CF3O radicals with selected alkenes and aromatics under atmospheric conditions

DEFF Research Database (Denmark)

Kelly, C.; Sidebottom, H.W.; Treacy, J.

1994-01-01

Rate data for the reactions of CF3O radicals with alkenes and aromatic compounds have been determined at 298 K using a relative rate method. The data are analyzed in terms of structure-reactivity relationships, and their importance to the atmospheric chemistry of CF3O discussed.......Rate data for the reactions of CF3O radicals with alkenes and aromatic compounds have been determined at 298 K using a relative rate method. The data are analyzed in terms of structure-reactivity relationships, and their importance to the atmospheric chemistry of CF3O discussed....

An iron/amine-catalyzed cascade process for the enantioselective functionalization of allylic alcohols.

Science.gov (United States)

Quintard, Adrien; Constantieux, Thierry; Rodriguez, Jean

2013-12-02

Three is a lucky number: An enantioselective transformation of allylic alcohols into β-chiral saturated alcohols has been developed by combining two distinct metal- and organocatalyzed catalytic cycles. This waste-free triple cascade process merges an iron-catalyzed borrowing-hydrogen step with an aminocatalyzed nucleophilic addition reaction. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Experimental study of the reactions of limonene with OH and OD radicals: kinetics and products.

Science.gov (United States)

Braure, Tristan; Bedjanian, Yuri; Romanias, Manolis N; Morin, Julien; Riffault, Véronique; Tomas, Alexandre; Coddeville, Patrice

2014-10-09

The kinetics of the reactions of limonene with OH and OD radicals has been studied using a low-pressure flow tube reactor coupled with a quadrupole mass spectrometer: OH + C10H16 → products (1), OD + C10H16 → products (2). The rate constants of the title reactions were determined using four different approaches: either monitoring the kinetics of OH (OD) radicals or limonene consumption in excess of limonene or of the radicals, respectively (absolute method), and by the relative rate method using either the reaction OH (OD) + Br2 or OH (OD) + DMDS (dimethyl disulfide) as the reference one and following HOBr (DOBr) formation or DMDS and limonene consumption, respectively. As a result of the absolute and relative measurements, the overall rate coefficients, k1 = (3.0 ± 0.5) × 10(-11) exp((515 ± 50)/T) and k2 = (2.5 ± 0.6) × 10(-11) exp((575 ± 60)/T) cm(3) molecule(-1) s(-1), were determined at a pressure of 1 Torr of helium over the temperature ranges 220-360 and 233-353 K, respectively. k1 was found to be pressure independent over the range 0.5-5 Torr. There are two possible pathways for the reaction between OH (OD) and limonene: addition of the radical to one of the limonene double bonds (reactions 1a and 2a ) and abstraction of a hydrogen atom (reactions 1b and 2b ), resulting in the formation of H2O (HOD). Measurements of the HOD yield as a function of temperature led to the following branching ratio of the H atom abstraction channel: k2b/k2 = (0.07 ± 0.03) × exp((460 ± 140)/T) for T = (253-355) K.

A paradigm shift for radical SAM reactions: The organometallic intermediate Ω is central to catalysis.

Science.gov (United States)

Byer, Amanda S; Yang, Hao; McDaniel, Elizabeth C; Kathiresan, Venkatesan; Impano, Stella; Pagnier, Adrien; Watts, Hope; Denler, Carly; Vagstad, Anna; Piel, Jörn; Duschene, Kaitlin S; Shepard, Eric M; Shields, Thomas P; Scott, Lincoln G; Lilla, Edward A; Yokoyama, Kenichi; Broderick, William E; Hoffman, Brian M; Broderick, Joan B

2018-06-28

Radical S-adenosyl-L-methionine (SAM) en-zymes comprise a vast superfamily catalyzing diverse reactions essential to all life through ho-molytic SAM cleavage to liberate the highly-reactive 5-deoxyadenosyl radical (5-dAdo•). Our recent observation of a catalytically compe-tent organometallic intermediate Ω that forms dur-ing reaction of the radical SAM (RS) enzyme py-ruvate formate-lyase activating-enzyme (PFL-AE) was therefore quite surprising, and led to the question of its broad relevance in the superfamily. We now show that Ω in PFL-AE forms as an in-termediate under a variety of mixing order condi-tions, suggesting it is central to catalysis in this enzyme. We further demonstrate that Ω forms in a suite of RS enzymes chosen to span the totality of superfamily reaction types, implicating Ω as essential in catalysis across the RS superfamily. Finally, EPR and electron nuclear double reso-nance spectroscopy establish that Ω involves an Fe-C5 bond between 5-dAdo• and the [4Fe-4S] cluster. An analogous organometallic bond is found in the well-known adenosylcobalamin (co-enzyme B12) cofactor used to initiate radical reac-tions via a 5'-dAdo• intermediate. Generation of a 5'-dAdo• intermediate via homolytic metal-carbon bond cleavage thus appears to be similar for Ω and coenzyme B12. However coenzyme B12 is involved in enzymes catalyzing of only a small number (~12) of distinct reactions, while the RS superfamily has more than 100,000 distinct se-quences and over 80 reaction types character-ized to date. The appearance of Ω across the RS superfamily therefore dramatically enlarges the sphere of bio-organometallic chemistry in Nature.

Kinetic and dynamic kinetic resolution of secondary alcohols with ionic-surfactant-coated Burkholderia cepacia lipase: substrate scope and enantioselectivity.

Science.gov (United States)

Kim, Cheolwoo; Lee, Jusuk; Cho, Jeonghun; Oh, Yeonock; Choi, Yoon Kyung; Choi, Eunjeong; Park, Jaiwook; Kim, Mahn-Joo

2013-03-15

Forty-four different secondary alcohols, which can be classified into several types (II-IX), were tested as the substrates of ionic surfactant-coated Burkholderia cepacia lipase (ISCBCL) to see its substrate scope and enantioselectivity in kinetic and dynamic kinetic resolution (KR and DKR). They include 6 boron-containing alcohols, 24 chiral propargyl alcohols, and 14 diarylmethanols. The results from the studies on KR indicate that ISCBCL accepted most of them with high enantioselectivity at ambient temperature and with useful to high enantioselectivity at elevated temperatures. In particular, ISCBCL displayed high enantioselectivity toward sterically demanding secondary alcohols (types VIII and IX) which have two bulky substituents at the hydroxymethine center. DKR reactions were performed by the combination of ISCBCL with a ruthenium-based racemization catalyst at 25-60 °C. Forty-one secondary alcohols were tested for DKR. About half of them were transformed into their acetates of high enantiopurity (>90% ee) with good yields (>80%). It is concluded that ISCBCL appears to be a superb enzyme for the KR and DKR of secondary alcohols.

Predicting CYP2C19 Catalytic Parameters for Enantioselective Oxidations Using Artificial Neural Networks and a Chirality Code

Science.gov (United States)

Hartman, Jessica H.; Cothren, Steven D.; Park, Sun-Ha; Yun, Chul-Ho; Darsey, Jerry A.; Miller, Grover P.

2013-01-01

Cytochromes P450 (CYP for isoforms) play a central role in biological processes especially metabolism of chiral molecules; thus, development of computational methods to predict parameters for chiral reactions is important for advancing this field. In this study, we identified the most optimal artificial neural networks using conformation-independent chirality codes to predict CYP2C19 catalytic parameters for enantioselective reactions. Optimization of the neural networks required identifying the most suitable representation of structure among a diverse array of training substrates, normalizing distribution of the corresponding catalytic parameters (kcat, Km, and kcat/Km), and determining the best topology for networks to make predictions. Among different structural descriptors, the use of partial atomic charges according to the CHelpG scheme and inclusion of hydrogens yielded the most optimal artificial neural networks. Their training also required resolution of poorly distributed output catalytic parameters using a Box-Cox transformation. End point leave-one-out cross correlations of the best neural networks revealed that predictions for individual catalytic parameters (kcat and Km) were more consistent with experimental values than those for catalytic efficiency (kcat/Km). Lastly, neural networks predicted correctly enantioselectivity and comparable catalytic parameters measured in this study for previously uncharacterized CYP2C19 substrates, R- and S-propranolol. Taken together, these seminal computational studies for CYP2C19 are the first to predict all catalytic parameters for enantioselective reactions using artificial neural networks and thus provide a foundation for expanding the prediction of cytochrome P450 reactions to chiral drugs, pollutants, and other biologically active compounds. PMID:23673224

The preparation and intramolecular radical cyclisation reactions of chiral oxyme ethers

International Nuclear Information System (INIS)

Booth, Susan E.; Jenkins, Paul R.

1998-01-01

Chiral oxime ether 2 and Oxime ester 4 have been prepared by alkylation and esterification of the oxime 1. Racemic hydroxylamine 6 and chiral hydroxylamine 10 have been synthesised from N-hydroxysuccinimide and the corresponding alcohol in the presence of diethyl azo dicarboxylate, the two product were converted into the oxime ethers 7 and 11 respectively. The intramolecular radical cyclisation reactions of these oxime ethers and esters has been studied, successful reaction was observed to produce alkyl hydroxylamines 3,8 and 12. (author)

The Preparation and Intramolecular Radical Cyclisation Reactions of Chiral Oxime Ethers

Directory of Open Access Journals (Sweden)

Booth Susan E.

1998-01-01

Full Text Available Chiral oxime ether 2 and Oxime ester 4 have been prepared by alkylation and esterification of the oxime 1. Racemic hydroxylamine 6 and chiral hydroxylamine 10 have been synthesised from N-hydroxysuccinimide and the corresponding alcohol in the presence of diethylazodicarboxylate, the two products were converted into the oxime ethers 7 and 11 respectively. The intramolecular radical cyclisation reactions of these oxime ethers and esters has been studied, successful reaction was observed to produce alkyl hydroxylamines 3, 8 and 12.

Methane coupling reaction in an oxy-steam stream through an OH radical pathway by using supported alkali metal catalysts

KAUST Repository

Liang, Yin

2014-03-24

A universal reaction mechanism involved in the oxidative coupling of methane (OCM) is demonstrated under oxy-steam conditions using alkali-metal-based catalysts. Rigorous kinetic measurements indicated a reaction mechanism that is consistent with OH radical formation from a H 2O-O2 reaction followed by C-H activation in CH 4 with an OH radical. Thus, the presence of water enhances both the CH4 conversion rate and the C2 selectivity. This OH radical pathway that is selective for the OCM was observed for the catalyst without Mn, which suggests clearly that Mn is not the essential component in a selective OCM catalyst. The experiments with different catalyst compositions revealed that the OH.-mediated pathway proceeded in the presence of catalysts with different alkali metals (Na, K) and different oxo anions (W, Mo). This difference in catalytic activity for OH radical generation accounts for the different OCM selectivities. As a result, a high C2 yield is achievable by using Na2WO4/SiO2, which catalyzes the OH.-mediated pathway selectively. Make it methane: A universal reaction mechanism involved in the oxidative coupling of methane is demonstrated under oxy-stream conditions by using alkali-metal-based catalysts. Rigorous kinetic measurements indicated a reaction mechanism that is consistent with OH radical formation from an H2O-O2 reaction, followed by C-H activation in CH4 with an OH radical. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Chiral amides via copper-catalysed enantioselective conjugate addition

NARCIS (Netherlands)

Schoonen, Anne K.; Fernández-Ibáñez, M. Ángeles; Fañanás-Mastral, Martín; Teichert, Johannes F.; Feringa, Bernard

2014-01-01

A highly enantioselective one pot procedure for the synthesis of beta-substituted amides was developed starting from the corresponding alpha,beta-unsaturated esters. This new methodology is based on the copper-catalysed enantioselective conjugate addition of Grignard reagents to

Tandem radical reactions and ring-closing metathesis. Application in the synthesis of cyclooctenes.

Science.gov (United States)

Sibi, Mukund P; Aasmul, Mona; Hasegawa, Hikaru; Subramanian, Thangaiah

2003-08-07

[reaction: see text] Fumarate- and acrylate-substituted oxazolidinones undergo tandem radical reaction to form dienes in moderate to good yields. The resulting dienes provide cyclooctenes in moderate to good yields after ring-closing metathesis (RCM). The role of the carbon backbone substituents and other variables in the efficiency of the eight-membered ring formation is discussed.

Atmospheric reactions of methylcyclohexanes with Cl atoms and OH radicals: determination of rate coefficients and degradation products.

Science.gov (United States)

Ballesteros, Bernabé; Ceacero-Vega, Antonio A; Jiménez, Elena; Albaladejo, José

2015-04-01

As the result of biogenic and anthropogenic activities, large quantities of chemical compounds are emitted into the troposphere. Alkanes, in general, and cycloalkanes are an important chemical class of hydrocarbons found in diesel, jet and gasoline, vehicle exhaust emissions, and ambient air in urban areas. In general, the primary atmospheric fate of organic compounds in the gas phase is the reaction with hydroxyl radicals (OH). The oxidation by Cl atoms has gained importance in the study of atmospheric reactions because they may exert some influence in the boundary layer, particularly in marine and coastal environments, and in the Arctic troposphere. The aim of this paper is to study of the atmospheric reactivity of methylcylohexanes with Cl atoms and OH radicals under atmospheric conditions (in air at room temperature and pressure). Relative kinetic techniques have been used to determine the rate coefficients for the reaction of Cl atoms and OH radicals with methylcyclohexane, cis-1,4-dimethylcyclohexane, trans-1,4-dimethylcyclohexane, and 1,3,5-trimethylcyclohexane at 298 ± 2 K and 720 ± 5 Torr of air by Fourier transform infrared) spectroscopy and gas chromatography-mass spectrometry (GC-MS) in two atmospheric simulation chambers. The products formed in the reaction under atmospheric conditions were investigated using a 200-L Teflon bag and employing the technique of solid-phase microextraction coupled to a GC-MS. The rate coefficients obtained for the reaction of Cl atoms with the studied compounds are the following ones (in units of 10(-10) cm(3) molecule(-1) s(-1)): (3.11 ± 0.16), (2.89 ± 0.16), (2.89 ± 0.26), and (2.61 ± 0.42), respectively. For the reactions with OH radicals the determined rate coefficients are (in units of 10(-11) cm(3) molecule(-1) s(-1)): (1.18 ± 0.12), (1.49 ± 0.16), (1.41 ± 0.15), and (1.77 ± 0.23), respectively. The reported error is twice the standard deviation. A detailed

Atmospheric chemistry of CF3COOH. Kinetics of the reaction with OH radicals

DEFF Research Database (Denmark)

Møgelberg, T.E.; Nielsen, O.J.; Sehested, J.

1994-01-01

Two different experimental techniques were used to study the kinetics of the reaction of OH radicals with trifluoroacetic acid, CF3COOH. Using a pulse radiolysis absolute rate technique, rate constants at 315 and 348 K were determined to be (1.6 +/- 0.4) x 10(-13) and (1.5 +/- 0.2) x 10(-13) cm3...... molecule-1 s-1, respectively. Using a long path-length FTIR relative rate technique a rate constant of (1.7 +/- 0.5) x 10(-13) cm3 molecule-1 s-1 was obtained at 296 K. In the atmosphere, reaction with OH radicals in the gas phase is estimated to account for 10%-20% of the loss of CF3COOH. The major fate...

Theoretical study on the mechanism of the reaction of FOX-7 with OH and NO2 radicals: bimolecular reactions with low barrier during the decomposition of FOX-7

Science.gov (United States)

Zhang, Ji-Dong; Zhang, Li-Li

2017-12-01

The decomposition of 1,1-diamino-2,2-dinitroethene (FOX-7) attracts great interests, while the studies on bimolecular reactions during the decomposition of FOX-7 are scarce. This study for the first time investigated the bimolecular reactions of OH and NO2 radicals, which are pyrolysis products of ammonium perchlorate (an efficient oxidant usually used in solid propellant), with FOX-7 by computational chemistry methods. The molecular geometries and energies were calculated using the (U)B3LYP/6-31++G(d,p) method. The rate constants of the reactions were calculated by canonical variational transition state theory. We found three mechanisms (H-abstraction, OH addition to C and N atom) for the reaction of OH + FOX-7 and two mechanisms (O abstraction and H abstraction) for the reaction of NO2 + FOX-7. OH radical can abstract H atom or add to C atom of FOX-7 with barriers near to zero, which means OH radical can effectively degrade FOX-7. The O abstraction channel of the reaction of NO2 + FOX-7 results in the formation of NO3 radical, which has never been detected experimentally during the decomposition of FOX-7.

Chemo- and Enantioselective Intramolecular Silver-Catalyzed Aziridinations.

Science.gov (United States)

Ju, Minsoo; Weatherly, Cale D; Guzei, Ilia A; Schomaker, Jennifer M

2017-08-07

Asymmetric nitrene-transfer reactions are a powerful tool for the preparation of enantioenriched amine building blocks. Reported herein are chemo- and enantioselective silver-catalyzed aminations which transform di- and trisubstituted homoallylic carbamates into [4.1.0]-carbamate-tethered aziridines in good yields and with ee values of up to 92 %. The effects of the substrate, silver counteranion, ligand, solvent, and temperature on both the chemoselectivity and ee value were explored. Stereochemical models were proposed to rationalize the observed absolute stereochemistry of the aziridines, which undergo nucleophilic ring opening to yield enantioenriched amines with no erosion in stereochemical integrity. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Exploring the dynamics of hydrogen atom release from the radical-radical reaction of O(3P) with C3H5

International Nuclear Information System (INIS)

Joo, Sun-Kyu; Kwon, Lee-Kyoung; Lee, Hohjai; Choi, Jong-Ho

2004-01-01

The gas-phase radical-radical reaction dynamics of O( 3 P)+C 3 H 5 →H( 2 S)+C 3 H 4 O was studied at an average collision energy of 6.4 kcal/mol in a crossed beam configuration. The ground-state atomic oxygen [O( 3 P)] and allyl radicals (C 3 H 5 ) were generated by the photolysis of NO 2 and the supersonic flash pyrolysis of allyl iodide, respectively. Nascent hydrogen atom products were probed by the vacuum-ultraviolet-laser induced fluorescence spectroscopy in the Lyman-α region centered at 121.6 nm. With the aid of the CBS-QB3 level of ab initio theory, it has been found that the barrierless addition of O( 3 P) to C 3 H 5 forms the energy-rich addition complexes on the lowest doublet potential energy surface, which are predicted to undergo a subsequent direct decomposition step leading to the reaction products H+C 3 H 4 O. The major counterpart C 3 H 4 O of the probed hydrogen atom is calculated to be acrolein after taking into account the factors of barrier height, reaction enthalpy, and the number of intermediates involved along the reaction pathway. The nascent H-atom Doppler profile analysis shows that the average center-of-mass translational energy of the H+C 3 H 4 O products and the fraction of the total available energy released as the translational energy were determined to be 3.83 kcal/mol and 0.054, respectively. On the basis of comparison with statistical calculations, the reaction proceeds through the formation of short-lived addition complexes rather than statistical, long-lived intermediates, and the polyatomic acrolein product is significantly internally excited at the moment of the decomposition

Exploring the dynamics of hydrogen atom release from the radical-radical reaction of O(3P) with C3H5

Science.gov (United States)

Joo, Sun-Kyu; Kwon, Lee-Kyoung; Lee, Hohjai; Choi, Jong-Ho

2004-05-01

The gas-phase radical-radical reaction dynamics of O(3P)+C3H5→H(2S)+C3H4O was studied at an average collision energy of 6.4 kcal/mol in a crossed beam configuration. The ground-state atomic oxygen [O(3P)] and allyl radicals (C3H5) were generated by the photolysis of NO2 and the supersonic flash pyrolysis of allyl iodide, respectively. Nascent hydrogen atom products were probed by the vacuum-ultraviolet-laser induced fluorescence spectroscopy in the Lyman-α region centered at 121.6 nm. With the aid of the CBS-QB3 level of ab initio theory, it has been found that the barrierless addition of O(3P) to C3H5 forms the energy-rich addition complexes on the lowest doublet potential energy surface, which are predicted to undergo a subsequent direct decomposition step leading to the reaction products H+C3H4O. The major counterpart C3H4O of the probed hydrogen atom is calculated to be acrolein after taking into account the factors of barrier height, reaction enthalpy, and the number of intermediates involved along the reaction pathway. The nascent H-atom Doppler profile analysis shows that the average center-of-mass translational energy of the H+C3H4O products and the fraction of the total available energy released as the translational energy were determined to be 3.83 kcal/mol and 0.054, respectively. On the basis of comparison with statistical calculations, the reaction proceeds through the formation of short-lived addition complexes rather than statistical, long-lived intermediates, and the polyatomic acrolein product is significantly internally excited at the moment of the decomposition.

Reaction kinetics of hydrogen atom abstraction from isopentanol by the H atom and HO2˙ radical.

Science.gov (United States)

Parab, Prajakta Rajaram; Heufer, K Alexander; Fernandes, Ravi Xavier

2018-04-25

Isopentanol is a potential next-generation biofuel for future applications to Homogeneous Charge Compression Ignition (HCCI) engine concepts. To provide insights into the combustion behavior of isopentanol, especially to its auto-ignition behavior which is linked both to efficiency and pollutant formation in real combustion systems, detailed quantum chemical studies for crucial reactions are desired. H-Abstraction reaction rates from fuel molecules are key initiation steps for chain branching required for auto-ignition. In this study, rate constants are determined for the hydrogen atom abstraction reactions from isopentanol by the H atom and HO2˙ radical by implementing the CBS-QB3 composite method. For the treatment of the internal rotors, a Pitzer-Gwinn-like approximation is applied. On comparing the computed reaction energies, the highest exothermicity (ΔE = -46 kJ mol-1) is depicted for Hα abstraction by the H atom whereas the lowest endothermicity (ΔE = 29 kJ mol-1) is shown for the abstraction of Hα by the HO2˙ radical. The formation of hydrogen bonding is found to affect the kinetics of the H atom abstraction reactions by the HO2˙ radical. Further above 750 K, the calculated high pressure limit rate constants indicate that the total contribution from delta carbon sites (Cδ) is predominant for hydrogen atom abstraction by the H atom and HO2˙ radical.

A Simple Primary Amine Catalyst for Enantioselective α-Hydroxylations and α-Fluorinations of Branched Aldehydes

OpenAIRE

Witten, Michael R.; Jacobsen, Eric N.

2015-01-01

A new primary amine catalyst for the asymmetric α-hydroxylation and α-fluorination of α-branched aldehydes is described. The products of the title transformations are generated in excellent yields and with high enantioselectivities. Both processes can be performed within short reaction times and on gram scale. The similarity in the results obtained in both reactions, combined with computational evidence, implies a common basis for stereoinduction and the possibility of a general catalytic mec...

A combined high-temperature experimental and theoretical kinetic study of the reaction of dimethyl carbonate with OH radicals

KAUST Repository

Khaled, Fathi

2017-02-08

The reaction kinetics of dimethyl carbonate (DMC) and OH radicals were investigated behind reflected shock waves over the temperature range of 872-1295 K and at pressures near 1.5 atm. Reaction progress was monitored by detecting OH radicals at 306.69 nm using a UV laser absorption technique. The rate coefficients for the reaction of DMC with OH radicals were extracted using a detailed kinetic model developed by Glaude et al. (Proc. Combust. Inst. 2005, 30(1), 1111-1118). The experimental rate coefficients can be expressed in Arrhenius form as: kexpt\\'l = 5.15 × 10(13) exp(-2710.2/T) cm(3) mol(-1) s(-1). To explore the detailed chemistry of the DMC + OH reaction system, theoretical kinetic analyses were performed using high-level ab initio and master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) calculations. Geometry optimization and frequency calculations were carried out at the second-order Møller-Plesset (MP2) perturbation level of theory using Dunning\\'s augmented correlation consistent-polarized valence double-ζ basis set (aug-cc-pVDZ). The energy was extrapolated to the complete basis set using single point calculations performed at the CCSD(T)/cc-pVXZ (where X = D, T) level of theory. For comparison purposes, additional ab initio calculations were also carried out using composite methods such as CBS-QB3, CBS-APNO, G3 and G4. Our calculations revealed that the H-abstraction reaction of DMC by OH radicals proceeds via an addition elimination mechanism in an overall exothermic process, eventually forming dimethyl carbonate radicals and H2O. Theoretical rate coefficients were found to be in excellent agreement with those determined experimentally. Rate coefficients for the DMC + OH reaction were combined with literature rate coefficients of four straight chain methyl ester + OH reactions to extract site-specific rates of H-abstraction from methyl esters by OH radicals.

Regio- and Enantioselective N-Allylations of Imidazole, Benzimidazole, and Purine Heterocycles Catalyzed by Single-Component Metallacyclic Iridium Complexes

Science.gov (United States)

Stanley, Levi M.

2010-01-01

Highly regio- and enantioselective iridium-catalyzed N-allylations of benzimidazoles, imidazoles, and purines have been developed. N-Allylated benzimidazoles and imidazoles were isolated in high yields (up to 97%) with high branched-to-linear selectivity (up to 99:1) and enantioselectivity (up to 98% ee) from the reactions of benzimidazole and imidazole nucleophiles with unsymmetrical allylic carbonates in the presence of single component, ethylene-bound, metallacyclic iridium catalysts. N-Allylated purines were also obtained in high yields (up to 91%) with high N9:N7 selectivity (up to 96:4), high branched-to-linear selectivity (98:2), and high enantioselectivity (up to 98% ee) under similar conditions. The reactions encompass a range of benzimidazole, imidazole, and purine nucleophiles, as well as a variety of unsymmetrical aryl, heteroaryl, and aliphatic allylic carbonates. Competition experiments between common amine nucleophiles and the heterocyclic nitrogen nucleophiles studied in this work illustrate the effect of nucleophile pKa on the rate of iridium-catalyzed N-allylation reactions. Kinetic studies on the allylation of benzimidazole catalyzed by metallacyclic iridium-phosphoramidite complexes, in combination with studies on the deactivation of these catalysts in the presence of heterocyclic nucleophiles, provide insight into the effects of the structure of the phosphoramidite ligands on the stability of the metallacyclic catalysts. The data obtained from these studies has led to the development of N-allylations of benzimidazoles and imidazoles in the absence of an exogenous base. PMID:19480431

A computational study on Lewis acid-catalyzed diastereoselective acyclic radical allylation reactions with unusual selectivity dependence on temperature and epimer precursor.

Science.gov (United States)

Georgieva, Miglena K; Santos, A Gil

2014-12-05

In stereoselective radical reactions, it is accepted that the configuration of the radical precursor has no impact on the levels of stereoinduction, as a prochiral radical intermediate is planar, with two identical faces, independently of its origin. However, Sibi and Rheault (J. Am. Chem. Soc. 2000, 122, 8873-8879) remarkably obtained different selectivities in the trapping of radicals originated from two epimeric bromides, catalyzed by chelating Lewis acids. The selectivity rationalization was made on the basis of different conformational properties of each epimer. However, in this paper we show that the two epimers have similar conformational properties, which implies that the literature proposal is unable to explain the experimental results. We propose an alternative mechanism, in which the final selectivity is dependent on different reaction rates for radical formation from each epimer. By introducing a different perspective of the reaction mechanism, our model also allows the rationalization of different chemical yields obtained from each epimer, a result not rationalized by the previous model. Adaptation to other radical systems, under different reaction conditions, is also possible.

Reaction between peroxynitrite and boronates: EPR spin-trapping, HPLC analyses, and quantum mechanical study of the free radical pathway

Science.gov (United States)

Sikora, Adam; Zielonka, Jacek; Lopez, Marcos; Dybala-Defratyka, Agnieszka; Joseph, Joy; Marcinek, Andrzej; Kalyanaraman, Balaraman

2013-01-01

Recently we showed that peroxynitrite (ONOO−) reacts directly and rapidly with aromatic and aliphatic boronic acids (k ≈ 106 M−1s−1). Product analyses and substrate consumption data indicated that ONOO− reacts stoichiometrically with boronates, yielding the corresponding phenols as the major product (~85–90%), and the remaining products (10–15%) were proposed to originate from free radical intermediates (phenyl and phenoxyl radicals). Here we investigated in detail the minor, free radical pathway of boronate reaction with ONOO−. The electron paramagnetic resonance (EPR) spin-trapping technique was used to characterize the free radical intermediates formed from the reaction between boronates and ONOO−. Using 2-methyl-2-nitrosopropane (MNP) and 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO) spin traps, phenyl radicals were trapped and detected. Although phenoxyl radicals were not detected, the positive effects of molecular oxygen, and inhibitory effects of hydrogen atom donors (acetonitrile, and 2-propanol) and general radical scavengers (GSH, NADH, ascorbic acid and tyrosine) on the formation of phenoxyl radical-derived nitrated product, suggest that phenoxyl radical was formed as the secondary species. We propose that the initial step of the reaction involves the addition of ONOO− to the boron atom in boronates. The anionic intermediate undergoes both heterolytic (major pathway) and homolytic (minor pathway) cleavage of the peroxy (O-O) bond to form phenol and nitrite as a major product (via a non-radical mechanism), or a radical pair PhB(OH)2O•−…•NO2 as a minor product. It is conceivable that phenyl radicals are formed by the fragmentation of PhB(OH)2O•− radical anion. According to the DFT quantum mechanical calculations, the energy barrier for the dissociation of PhB(OH)2O•− radical anion to form phenyl radicals is only a few kcal/mol, suggesting rapid and spontaneous fragmentation of PhB(OH)2O•− radical anion

Modeling of the Reaction Mechanism of Enzymatic Radical C–C Coupling by Benzylsuccinate Synthase

Directory of Open Access Journals (Sweden)

Maciej Szaleniec

2016-04-01

Full Text Available Molecular modeling techniques and density functional theory calculations were performed to study the mechanism of enzymatic radical C–C coupling catalyzed by benzylsuccinate synthase (BSS. BSS has been identified as a glycyl radical enzyme that catalyzes the enantiospecific fumarate addition to toluene initiating its anaerobic metabolism in the denitrifying bacterium Thauera aromatica, and this reaction represents the general mechanism of toluene degradation in all known anaerobic degraders. In this work docking calculations, classical molecular dynamics (MD simulations, and DFT+D2 cluster modeling was employed to address the following questions: (i What mechanistic details of the BSS reaction yield the most probable molecular model? (ii What is the molecular basis of enantiospecificity of BSS? (iii Is the proposed mechanism consistent with experimental observations, such as an inversion of the stereochemistry of the benzylic protons, syn addition of toluene to fumarate, exclusive production of (R-benzylsuccinate as a product and a kinetic isotope effect (KIE ranging between 2 and 4? The quantum mechanics (QM modeling confirms that the previously proposed hypothetical mechanism is the most probable among several variants considered, although C–H activation and not C–C coupling turns out to be the rate limiting step. The enantiospecificity of the enzyme seems to be enforced by a thermodynamic preference for binding of fumarate in the pro(R orientation and reverse preference of benzyl radical attack on fumarate in pro(S pathway which results with prohibitively high energy barrier of the radical quenching. Finally, the proposed mechanism agrees with most of the experimental observations, although the calculated intrinsic KIE from the model (6.5 is still higher than the experimentally observed values (4.0 which suggests that both C–H activation and radical quenching may jointly be involved in the kinetic control of the reaction.

Thermal stability of carbonyl radicals. Part II. Reactions of methylglyoxyl and methylglyoxylperoxy radicals at 1 bar in the temperature range 275-311 K.

Science.gov (United States)

Jagiella, Stefan; Zabel, Friedhelm

2008-04-07

Reactions of methylglyoxyl and methylglyoxylperoxy radicals were investigated at a total pressure of 1 bar in oxygen. Methylglyoxyl radicals were generated by stationary photolysis of Br2-CH3C(O)C(O)H-NO2-O2-N2 mixtures at wavelengths > or =480 nm and of Cl2-CH3C(O)C(O)H-NO2-O2-N2 mixtures in the wavelength range 315-460 nm. In the bromine system, rate constant ratios for the reactions CH3C(O)CO --> CH3CO + CO (kdis) and CH3C(O)CO + O2 --> CH3C(O)C(O)O2 (kO2) were measured as a function of temperature in the range 275-311 K. Assuming the constant value kO2 = 5.1 x 10(-12) cm3 molecule(-1) s(-1) for our reaction conditions, kdis = 1.2 x 10(10.0+/-0.7) x exp(-11.7 +/- 3.8 kJ mol(-1)/RT) s(-1) (2sigma errors) was obtained for ptot = 1 bar (M = O2), in good agreement with the kinetic parameters calculated by Méreau et al. [R. Méreau, M.-T. Rayez, J.-C. Rayez, F. Caralp and R. Lesclaux, Phys. Chem. Chem. Phys., 2001, 3, 4712]. CH3C(O)C(O)O2 radicals oxidise NO2, forming NO3, CH3CO and CO2. This experimental result is supported by DFT and ab initio calculations. Possible mechanisms for the observed formation of several % of ketene and bromoacetyl peroxynitrate are discussed. Use of Cl rather than Br atoms to abstract the aldehydic H atom from methylglyoxal leads to chemically activated CH3C(O)CO radicals, thus substantially increasing the fraction of CH3C(O)CO radicals that decompose rather than add O2.

Molecular Design of a Chiral Brønsted Acid with Two Different Acidic Sites: Regio-, Diastereo-, and Enantioselective Hetero-Diels-Alder Reaction of Azopyridinecarboxylate with Amidodienes Catalyzed by Chiral Carboxylic Acid-Monophosphoric Acid.

Science.gov (United States)

Momiyama, Norie; Tabuse, Hideaki; Noda, Hirofumi; Yamanaka, Masahiro; Fujinami, Takeshi; Yamanishi, Katsunori; Izumiseki, Atsuto; Funayama, Kosuke; Egawa, Fuyuki; Okada, Shino; Adachi, Hiroaki; Terada, Masahiro

2016-09-07

A chiral Brønsted acid containing two different acidic sites, chiral carboxylic acid-monophosphoric acid 1a, was designed to be a new and effective concept in catalytic asymmetric hetero-Diels-Alder reactions of azopyridinecarboxylate with amidodienes. The multipoint hydrogen-bonding interactions among the carboxylic acid, monophosphoric acid, azopyridinecarboxylate, and amidodiene achieved high catalytic and chiral efficiency, producing substituted 1,2,3,6-tetrahydropyridazines with excellent stereocontrol in a single step. This constitutes the first example of regio-, diastereo-, and enantioselective azo-hetero-Diels-Alder reactions by chiral Brønsted acid catalysis.

Diastereo- and enantioselective iridium-catalyzed allylation of cyclic ketone enolates: synergetic effect of ligands and barium enolates.

Science.gov (United States)

Chen, Wenyong; Chen, Ming; Hartwig, John F

2014-11-12

We report asymmetric allylic alkylation of barium enolates of cyclic ketones catalyzed by a metallacyclic iridium complex containing a phosphoramidite ligand derived from (R)-1-(2-naphthyl)ethylamine. The reaction products contain adjacent quaternary and tertiary stereocenters. This process demonstrates that unstabilized cyclic ketone enolates can undergo diastereo- and enantioselective Ir-catalyzed allylic substitution reactions with the proper choice of enolate countercation. The products of these reactions can be conveniently transformed to various useful polycarbocyclic structures.

A general enantioselective route to the chamigrene natural product family

KAUST Repository

White, David E.; Stewart, Ian C.; Seashore-Ludlow, Brinton A.; Grubbs, Robert H.; Stoltz, Brian M.

2010-01-01

Described in this report is an enantioselective route toward the chamigrene natural product family. The key disconnections in our synthetic approach include sequential enantioselective decarboxylative allylation and ring-closing olefin metathesis to form the all-carbon quaternary stereocenter and spirocyclic core present in all members of this class of compounds. The generality of this strategy is demonstrated by the first total syntheses of elatol and the proposed structure of laurencenone B, as well as the first enantioselective total syntheses of laurencenone C and α-chamigrene. A brief exploration of the substrate scope of the enantioselective decarboxylative allylation/ring-closing metathesis sequence with fully substituted vinyl chlorides is also presented.

A general enantioselective route to the chamigrene natural product family

KAUST Repository

White, David E.

2010-06-01

Described in this report is an enantioselective route toward the chamigrene natural product family. The key disconnections in our synthetic approach include sequential enantioselective decarboxylative allylation and ring-closing olefin metathesis to form the all-carbon quaternary stereocenter and spirocyclic core present in all members of this class of compounds. The generality of this strategy is demonstrated by the first total syntheses of elatol and the proposed structure of laurencenone B, as well as the first enantioselective total syntheses of laurencenone C and α-chamigrene. A brief exploration of the substrate scope of the enantioselective decarboxylative allylation/ring-closing metathesis sequence with fully substituted vinyl chlorides is also presented.

Enantioselective desymmetrization of prochiral cyclohexanones by organocatalytic intramolecular Michael additions to α,β-unsaturated esters.

Science.gov (United States)

Gammack Yamagata, Adam D; Datta, Swarup; Jackson, Kelvin E; Stegbauer, Linus; Paton, Robert S; Dixon, Darren J

2015-04-13

A new catalytic asymmetric desymmetrization reaction for the synthesis of enantioenriched derivatives of 2-azabicyclo[3.3.1]nonane, a key motif common to many alkaloids, has been developed. Employing a cyclohexanediamine-derived primary amine organocatalyst, a range of prochiral cyclohexanone derivatives possessing an α,β-unsaturated ester moiety linked to the 4-position afforded the bicyclic products, which possess three stereogenic centers, as single diastereoisomers in high enantioselectivity (83-99% ee) and in good yields (60-90%). Calculations revealed that stepwise C-C bond formation and proton transfer via a chair-shaped transition state dictate the exclusive endo selectivity and enabled the development of a highly enantioselective primary amine catalyst. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Time-resolved studies of free radicals and laser-initiated chain reactions: Final report, 1 April 1979-31 March 1988

International Nuclear Information System (INIS)

Leone, S.R.

1988-03-01

Pulsed lasers were used in this work to photofragment molecules or to initiate chain reactions. One of the major advances was the availability of high-powered rare gas halide excimer lasers. In addition, pulsed Nd:YAG lasers and dye lasers were used throughout. Results include: generalized kinetic formulations of the problem of laser-initiated chain reactions. Several studies were carried out to explore the details of chain combustion phenomena, slow chain reactions, chain branching behavior, and vibrational temperatures of combusting mixtures. A method to determine the rotational temperature of nitrogen molecules by laser multiphoton ionization was shown. The chain reaction methodology was applied to complex polyatomic systems, in which complete infrared spectra of the emitting species were obtained. Systems studied included, chlorine + HBr, HI, methane, hydrogen, ethane, propane, butane, cyclopropane, and cyclohexane. Photofragmentation studies involved the production and analysis of radical species, such as methyl, CH 2 I, and CCH. Molecules studied included methylene iodide, methyl iodide, dimethyl mercury, acetone, acetylene, vinyl chloride, dichloroethylene, and fluorochloroethylene. The first infrared characterization of a highly vibrationally excited radical was shown. Reactions of methyl radicals were studied in detail, in which a new method for obtaining absolute values of the methyl radical reaction rates were obtained

Enantioselective Diels-Alder Reaction Using Chiral Mg Complexes Derived from Chiral 2-[2-[(Alkyl- or 2-[2-[(Arylsulfonyl)amino]phenyl]-4-phenyl-1,3-oxazoline.

Science.gov (United States)

Ichiyanagi, Tsuyoshi; Shimizu, Makoto; Fujisawa, Tamotsu

1997-11-14

Magnesium complexes derived from (R)-2-[2-[(alkyl- or (R)-2-[2-[(arylsulfonyl)amino]phenyl]-4-phenyl-1,3-oxazolines and methylmagnesium iodide were found to be efficient Lewis acid catalysts for the Diels-Alder reaction of 3-alkenoyl-1,3-oxazolidin-2-one with cyclopentadiene. Chiral ligands were easily prepared from readily available D-phenylglycinol in good yields. The reaction of 3-acryloyl-1,3-oxazolidin-2-one with cyclopentadiene catalyzed by a stoichiometric amount of the Lewis acid gave exclusively the endo-cycloaddition product in up to 92% ee. The sulfonamide group on the chiral ligand strongly influenced the enantiofacial selectivity: the use of a toluene-, benzene-, 1- or 2-naphthalene-, or methanesulfonamide group in the chiral ligand gave the endo-(2R)-cycloaddition product, while a trifluoromethanesulfonamide group predominantly gave its enantiomer, the endo-(2S)-cycloaddition product, in 65% ee. The scope and limitations of the catalytic effect of chiral Mg(II) complexes on the enantioselectivity of the Diels-Alder reaction were investigated. The reaction mechanism of the Mg(II)-catalyzed reaction is also discussed on the basis of the experimental results.

Molecularly imprinted polymer-matrix nanocomposite for enantioselective electrochemical sensing of D- and L-aspartic acid

International Nuclear Information System (INIS)

Prasad, Bhim Bali; Srivastava, Amrita; Tiwari, Mahavir Prasad

2013-01-01

A new molecularly imprinted polymer-matrix (titanium dioxide nanoparticle/multiwalled carbon nanotubes) nanocomposite was developed for the modification of pencil graphite electrode as an enantioselective sensing probe for aspartic acid isomers, prevalent at ultra trace level in aqueous and real samples. The nanocomposite having many shape complementary cavities was synthesized adopting surface initiated-activators regenerated by electron transfer for atom transfer radical polymerization. The proposed sensor has high stability, nanocomposite uniformity, good reproducibility, and enhanced electrocatalytic activity to respond oxidative peak current of L-aspartic acid quantitatively by differential pulse anodic stripping voltammetry, without any cross-reactivity in real samples. Under the optimized operating conditions, the L-aspartic acid imprinted modified electrode showed a wide linear response for L-aspartic acid within the concentration range 9.98–532.72 ng mL −1 , with the minimum detection limit of 1.73–1.79 ng mL −1 (S/N = 3) in aqueous and real samples. Almost similar stringent limit (1.79 ng mL −1 ) was obtained with cerebrospinal fluid which is typical for the primitive diagnosis of neurological disorders, caused by an acute depletion of L-aspartic acid biomarker, in clinical settings. Highlights: • We have adopted surface initiated-activators regenerated by electron transfer for atom transfer radical polymerization. • This approach takes advantage of the nanostructured ultrathin imprinted film. • Successful enantioselective sensing and ultratrace analysis of D- and L-aspartic acid. • Stringent detection limit without any non-specific false-positive contribution

Molecularly imprinted polymer-matrix nanocomposite for enantioselective electrochemical sensing of D- and L-aspartic acid

Energy Technology Data Exchange (ETDEWEB)

Prasad, Bhim Bali, E-mail: prof.bbpd@yahoo.com; Srivastava, Amrita; Tiwari, Mahavir Prasad

2013-10-15

A new molecularly imprinted polymer-matrix (titanium dioxide nanoparticle/multiwalled carbon nanotubes) nanocomposite was developed for the modification of pencil graphite electrode as an enantioselective sensing probe for aspartic acid isomers, prevalent at ultra trace level in aqueous and real samples. The nanocomposite having many shape complementary cavities was synthesized adopting surface initiated-activators regenerated by electron transfer for atom transfer radical polymerization. The proposed sensor has high stability, nanocomposite uniformity, good reproducibility, and enhanced electrocatalytic activity to respond oxidative peak current of L-aspartic acid quantitatively by differential pulse anodic stripping voltammetry, without any cross-reactivity in real samples. Under the optimized operating conditions, the L-aspartic acid imprinted modified electrode showed a wide linear response for L-aspartic acid within the concentration range 9.98–532.72 ng mL{sup −1}, with the minimum detection limit of 1.73–1.79 ng mL{sup −1} (S/N = 3) in aqueous and real samples. Almost similar stringent limit (1.79 ng mL{sup −1}) was obtained with cerebrospinal fluid which is typical for the primitive diagnosis of neurological disorders, caused by an acute depletion of L-aspartic acid biomarker, in clinical settings. Highlights: • We have adopted surface initiated-activators regenerated by electron transfer for atom transfer radical polymerization. • This approach takes advantage of the nanostructured ultrathin imprinted film. • Successful enantioselective sensing and ultratrace analysis of D- and L-aspartic acid. • Stringent detection limit without any non-specific false-positive contribution.

Molecular weight growth in Titan's atmosphere: branching pathways for the reaction of 1-propynyl radical (H3CC≡C˙) with small alkenes and alkynes.

Science.gov (United States)

Kirk, Benjamin B; Savee, John D; Trevitt, Adam J; Osborn, David L; Wilson, Kevin R

2015-08-28

The reaction of small hydrocarbon radicals (i.e.˙CN, ˙C2H) with trace alkenes and alkynes is believed to play an important role in molecular weight growth and ultimately the formation of Titan's characteristic haze. Current photochemical models of Titan's atmosphere largely assume hydrogen atom abstraction or unimolecular hydrogen elimination reactions dominate the mechanism, in contrast to recent experiments that reveal significant alkyl radical loss pathways during reaction of ethynyl radical (˙C2H) with alkenes and alkynes. In this study, the trend is explored for the case of a larger ethynyl radical analogue, the 1-propynyl radical (H3CC[triple bond, length as m-dash]C˙), a likely product from the high-energy photolysis of propyne in Titan's atmosphere. Using synchrotron vacuum ultraviolet photoionization mass spectrometry, product branching ratios are measured for the reactions of 1-propynyl radical with a suite of small alkenes (ethylene and propene) and alkynes (acetylene and d4-propyne) at 4 Torr and 300 K. Reactions of 1-propynyl radical with acetylene and ethylene form single products, identified as penta-1,3-diyne and pent-1-en-3-yne, respectively. These products form by hydrogen atom loss from the radical-adduct intermediates. The reactions of 1-propynyl radical with d4-propyne and propene form products from both hydrogen atom and methyl loss, (-H = 27%, -CH3 = 73%) and (-H = 14%, -CH3 = 86%), respectively. Together, these results indicate that reactions of ethynyl radical analogues with alkenes and alkynes form significant quantities of products by alkyl loss channels, suggesting that current photochemical models of Titan over predict both hydrogen atom production as well as the efficiency of molecular weight growth in these reactions.

Enantioselective Biotransformation of Chiral Persistent Organic Pollutants.

Science.gov (United States)

Zhang, Ying; Ye, Jing; Liu, Min

2017-01-01

Enantiomers of chiral compounds commonly undergo enantioselective transformation in most biologically mediated processes. As chiral persistent organic pollutants (POPs) are extensively distributed in the environment, differences between enantiomers in biotransformation should be carefully considered to obtain exact enrichment and specific health risks. This review provides an overview of in vivo biotransformation of chiral POPs currently indicated in the Stockholm Convention and their chiral metabolites. Peer-reviewed journal articles focused on the research question were thoroughly searched. A set of inclusion and exclusion criteria were developed to identify relevant studies. We mainly compared the results from different animal models under controlled laboratory conditions to show the difference between enantiomers in terms of distinct transformation potential. Interactions with enzymes involved in enantioselective biotransformation, especially cytochrome P450 (CYP), were discussed. Further research areas regarding this issue were proposed. Limited evidence for a few POPs has been found in 30 studies. Enantioselective biotransformation of α-hexachlorocyclohexane (α-HCH), chlordane, dichlorodiphenyltrichloroethane (DDT), heptachlor, hexabromocyclododecane (HBCD), polychlorinated biphenyls (PCBs), and toxaphene, has been investigated using laboratory mammal, fish, bird, and worm models. Tissue and excreta distributions, as well as bioaccumulation and elimination kinetics after administration of racemate and pure enantiomers, have been analyzed in these studies. Changes in enantiomeric fractions have been considered as an indicator of enantioselective biotransformation of chiral POPs in most studies. Results of different laboratory animal models revealed that chiral POP biotransformation is seriously affected by chirality. Pronounced results of species-, tissue-, gender-, and individual-dependent differences are observed in in vivo biotransformation of chiral POPs

Photofragment imaging study of the CH2CCH2OH radical intermediate of the OH+allene reaction

International Nuclear Information System (INIS)

Raman, Arjun S.; Justine Bell, M.; Lau, K.-C.; Butler, Laurie J.

2007-01-01

These velocity map imaging experiments characterize the photolytic generation of one of the two radical intermediates formed when OH reacts via an addition mechanism with allene. The CH 2 CCH 2 OH radical intermediate is generated photolytically from the photodissociation of 2-chloro-2-propen-1-ol at 193 nm. Detecting the Cl atoms using [2+1] resonance-enhanced multiphoton ionization evidences an isotropic angular distribution for the Cl+CH 2 CCH 2 OH photofragments, a spin-orbit branching ratio for Cl( 2 P 1/2 ):Cl( 2 P 3/2 ) of 0.28, and a bimodal recoil kinetic energy distribution. Conservation of momentum and energy allows us to determine from this data the internal energy distribution of the nascent CH 2 CCH 2 OH radical cofragment. To assess the possible subsequent decomposition pathways of this highly vibrationally excited radical intermediate, we include electronic structure calculations at the G3//B3LYP level of theory. They predict the isomerization and dissociation transition states en route from the initial CH 2 CCH 2 OH radical intermediate to the three most important product channels for the OH+allene reaction expected from this radical intermediate: formaldehyde+C 2 H 3 , H+acrolein, and ethene+CHO. We also calculate the intermediates and transition states en route from the other radical adduct, formed by addition of the OH to the center carbon of allene, to the ketene+CH 3 product channel. We compare our results to a previous theoretical study of the O+allyl reaction conducted at the CBS-QB3 level of theory, as the two reactions include several common intermediates

Photofragment imaging study of the CH2CCH2OH radical intermediate of the OH +allene reaction

Science.gov (United States)

Raman, Arjun S.; Justine Bell, M.; Lau, Kai-Chung; Butler, Laurie J.

2007-10-01

These velocity map imaging experiments characterize the photolytic generation of one of the two radical intermediates formed when OH reacts via an addition mechanism with allene. The CH2CCH2OH radical intermediate is generated photolytically from the photodissociation of 2-chloro-2-propen-1-ol at 193nm. Detecting the Cl atoms using [2+1] resonance-enhanced multiphoton ionization evidences an isotropic angular distribution for the Cl +CH2CCH2OH photofragments, a spin-orbit branching ratio for Cl(P1/22):Cl(P3/22) of 0.28, and a bimodal recoil kinetic energy distribution. Conservation of momentum and energy allows us to determine from this data the internal energy distribution of the nascent CH2CCH2OH radical cofragment. To assess the possible subsequent decomposition pathways of this highly vibrationally excited radical intermediate, we include electronic structure calculations at the G3//B3LYP level of theory. They predict the isomerization and dissociation transition states en route from the initial CH2CCH2OH radical intermediate to the three most important product channels for the OH +allene reaction expected from this radical intermediate: formaldehyde+C2H3, H +acrolein, and ethene+CHO. We also calculate the intermediates and transition states en route from the other radical adduct, formed by addition of the OH to the center carbon of allene, to the ketene+CH3 product channel. We compare our results to a previous theoretical study of the O +allyl reaction conducted at the CBS-QB3 level of theory, as the two reactions include several common intermediates.

Poly(propylene carbonate): Insight into the Microstructure and Enantioselective Ring-Opening Mechanism

KAUST Repository

Salmeia, Khalifah A.

2012-11-13

Different poly(propylene carbonate) (PPC) microstructures have been synthesized from the alternating copolymerization of CO 2 with both racemic propylene oxide (PO) and various mixtures of PO enantiomers using chiral salen catalysts. The microstructures of the obtained copolymers as a function of polymerization time have been analyzed by a combination of chiral GC and high-resolution NMR spectroscopy. The 13C NMR spectra of selected poly(propylene carbonate) samples were recorded using a 900 MHz ( 1H) spectrometer, showing a previously unreported fine splitting of the carbonate resonances. This allowed a detailed assignment of signals for various copolymer microstructures taking into account the specifics in their stereo- and regioirregularities. For example, the enantioselectivity preference of the (R,R-salen)Co catalyst for (S)-PO at the beginning of the copolymerization leads predominantly to (S)-PO insertion, with any (R)-PO misinsertion being followed by incorporation of (S)-PO, so that the microstructure features isolated stereoerrors. K rel calculations for the copolymerization showed around 5-fold enantioselectivity for (S)-PO over (R)-PO at short reaction time. Analysis of the copolymer microstructures obtained under various reaction conditions appears to be an additional approach to differentiate the occurrence of bimetallic and bifunctional copolymerization mechanisms that are widely discussed in the literature. © 2012 American Chemical Society.

Poly(propylene carbonate): Insight into the Microstructure and Enantioselective Ring-Opening Mechanism

KAUST Repository

Salmeia, Khalifah A.; Vagin, Sergei; Anderson, Carly E.; Rieger, Bernhard

2012-01-01

Different poly(propylene carbonate) (PPC) microstructures have been synthesized from the alternating copolymerization of CO 2 with both racemic propylene oxide (PO) and various mixtures of PO enantiomers using chiral salen catalysts. The microstructures of the obtained copolymers as a function of polymerization time have been analyzed by a combination of chiral GC and high-resolution NMR spectroscopy. The 13C NMR spectra of selected poly(propylene carbonate) samples were recorded using a 900 MHz ( 1H) spectrometer, showing a previously unreported fine splitting of the carbonate resonances. This allowed a detailed assignment of signals for various copolymer microstructures taking into account the specifics in their stereo- and regioirregularities. For example, the enantioselectivity preference of the (R,R-salen)Co catalyst for (S)-PO at the beginning of the copolymerization leads predominantly to (S)-PO insertion, with any (R)-PO misinsertion being followed by incorporation of (S)-PO, so that the microstructure features isolated stereoerrors. K rel calculations for the copolymerization showed around 5-fold enantioselectivity for (S)-PO over (R)-PO at short reaction time. Analysis of the copolymer microstructures obtained under various reaction conditions appears to be an additional approach to differentiate the occurrence of bimetallic and bifunctional copolymerization mechanisms that are widely discussed in the literature. © 2012 American Chemical Society.

Singlet Oxygen and Free Radical Reactions of Retinoids and Carotenoids—A Review

Science.gov (United States)

Truscott, T. George

2018-01-01

We report on studies of reactions of singlet oxygen with carotenoids and retinoids and a range of free radical studies on carotenoids and retinoids with emphasis on recent work, dietary carotenoids and the role of oxygen in biological processes. Many previous reviews are cited and updated together with new data not previously reviewed. The review does not deal with computational studies but the emphasis is on laboratory-based results. We contrast the ease of study of both singlet oxygen and polyene radical cations compared to neutral radicals. Of particular interest is the switch from anti- to pro-oxidant behavior of a carotenoid with change of oxygen concentration: results for lycopene in a cellular model system show total protection of the human cells studied at zero oxygen concentration, but zero protection at 100% oxygen concentration. PMID:29301252

An experimental and theoretical kinetic study of the reaction of OH radicals with tetrahydrofuran

KAUST Repository

Giri, Binod

2016-06-24

Tetrahydrofuran (CHO, THF) and its alkylated derivatives of the cyclic ether family are considered to be promising future biofuels. They appear as important intermediates during the low-temperature oxidation of conventional hydrocarbon fuels and of heavy biofuels such as long-chain fatty acid methyl esters. The reaction of tetrahydrofuran with OH radicals was investigated in a shock tube, over a temperature range of 800-1340 K and at pressures near 1.5 bar. Hydroxyl radicals were generated by the rapid thermal decomposition of tert-butyl hydroperoxide, and a UV laser absorption technique was used to monitor the mole fraction of OH radicals. High-level CCSD(T)/cc-pV(D,T)Z//MP2/aug-cc-pVDZ quantum chemical calculations were performed to explore the chemistry of the THF+OH reaction system. Our calculations reveal that the THF+OH (R1) reaction proceeds via either direct or indirect H-abstraction from various sites, leading to the formation of tetrahydrofuran-2-yl (THF-R2) or tetrahydrofuran-3-yl (THF-R3) radicals and water. Theoretical kinetic analysis revealed that both channels are important under conditions relevant to combustion. To our knowledge, this is the first direct experimental and theoretical kinetic study of the reaction of tetrahydrofuran with OH radicals at high temperatures. The following theoretical rate expressions (in units of cmmols) are recommended for combustion modeling in the temperature range 800-1350 K: . k1(T)=4.11×1040.16em0ex(TK)2.69exp(1316.80.16em0exKT)2.em0ex0.16em0ex(THF+OH→Products) . k2(T)=6.930.16em0ex×10110.16em0ex(TK)0.41exp(-106.80.16em0exKT)2.em0ex0.16em0ex(THF+OH→THF-R20.16em0ex+H2O) . k3(T)=4.120.16em0ex×1030.16em0ex(TK)3.02exp(456.90.16em0exKT)2.em0ex0.16em0ex(THF+OH→THF-R30.16em0ex+H2O) . .

High-Throughput Screening of the Asymmetric Decarboxylative Alkylation Reaction of Enolate-Stabilized Enol Carbonates

KAUST Repository

Stoltz, Brian

2010-06-14

The use of high-throughput screening allowed for the optimization of reaction conditions for the palladium-catalyzed asymmetric decarboxylative alkylation reaction of enolate-stabilized enol carbonates. Changing to a non-polar reaction solvent and to an electron-deficient PHOX derivative as ligand from our standard reaction conditions improved the enantioselectivity for the alkylation of a ketal-protected,1,3-diketone-derived enol carbonate from 28% ee to 84% ee. Similar improvements in enantioselectivity were seen for a β-keto-ester derived- and an α-phenyl cyclohexanone-derived enol carbonate.

High-Throughput Screening of the Asymmetric Decarboxylative Alkylation Reaction of Enolate-Stabilized Enol Carbonates

KAUST Repository

Stoltz, Brian; McDougal, Nolan; Virgil, Scott

2010-01-01

The use of high-throughput screening allowed for the optimization of reaction conditions for the palladium-catalyzed asymmetric decarboxylative alkylation reaction of enolate-stabilized enol carbonates. Changing to a non-polar reaction solvent and to an electron-deficient PHOX derivative as ligand from our standard reaction conditions improved the enantioselectivity for the alkylation of a ketal-protected,1,3-diketone-derived enol carbonate from 28% ee to 84% ee. Similar improvements in enantioselectivity were seen for a β-keto-ester derived- and an α-phenyl cyclohexanone-derived enol carbonate.

Copper(II)-catalyzed enantioselective hydrosilylation of halo-substituted alkyl aryl and heteroaryl ketones: asymmetric synthesis of (R)-fluoxetine and (S)-duloxetine.

Science.gov (United States)

Zhou, Ji-Ning; Fang, Qiang; Hu, Yi-Hu; Yang, Li-Yao; Wu, Fei-Fei; Xie, Lin-Jie; Wu, Jing; Li, Shijun

2014-02-14

A set of reaction conditions has been established to facilitate the non-precious copper-catalyzed enantioselective hydrosilylation of a number of structurally diverse β-, γ- or ε-halo-substituted alkyl aryl ketones and α-, β- or γ-halo-substituted alkyl heteroaryl ketones under air to afford a broad spectrum of halo alcohols in high yields and good to excellent enantioselectivities (up to 99% ee). The developed procedure has been successfully applied to the asymmetric synthesis of antidepressant drugs (R)-fluoxetine and (S)-duloxetine, which highlighted its synthetic utility.

Reaction of long-lived radicals and vitamin C in γ-irradiated mammalian cells and their model system at 295 K. Tunneling reaction in biological system

International Nuclear Information System (INIS)

Matsumoto, Takuro; Kumada, Takayuki; Kodama, Seiji; Watanabe, Masami

1997-01-01

When golden hamster embryo (GHE) cells or concentrated albumin solution (0.1 kg dm -3 ), that is a model system of cells, is irradiated with γ-rays at 295 K, organic radicals produced can be observed by ESR. The organic radicals survive at both 295 and 310 K for as long as 20 h. The long-lived radicals in GHE cells and the albumin solution react with vitamin C by the rate constants of 0.007 dm 3 mol -1 s -1 and 0.014 dm 3 mol -1 s -1 , respectively. The long-lived radicals in human cells cause gene mutation, which is suppressed by the addition of vitamin C. The isotope effect on the rate constant (κ) for the reaction of the long-lived radicals and vitamin C has been studied in the albumin solution by use of protonated vitamin C and deuterated vitamin C. The isotope effect (κ H /κ D ) was more than 20 ∼ 50 and was interpreted in terms of tunnelling reaction. (author)

Products of aqueous vitamin B5 (pantothenic acid) formed by free radical reactions

International Nuclear Information System (INIS)

Schittl, H.; Quint, R.M.; Getoff, N.

2007-01-01

The radiolysis of aqueous vitamin B5 (pantothenic acid) has been investigated under various experimental conditions. The highest vitamin degradation (G=3.22) was observed in solutions saturated with N 2 O, where 90% OH radicals are operating. As final products, the following were established: aldehydes, carboxylic acids and ammonia. Their yield strongly depends on the presence/absence of air as well as on N 2 O (used to convert e aq - into OH) and was determined as a function of absorbed radiation dose. HPLC-analysis showed that in all media, a main product is formed, having the highest yield in aerated solutions. Based on the chemical analysis, it appears that the OH radicals are most involved in the degradation process. A precise sequence of the reaction steps could not be given presently, because of the implication of many simultaneous reactions

An efficient quantum mechanical method for radical pair recombination reactions.

Science.gov (United States)

Lewis, Alan M; Fay, Thomas P; Manolopoulos, David E

2016-12-28

The standard quantum mechanical expressions for the singlet and triplet survival probabilities and product yields of a radical pair recombination reaction involve a trace over the states in a combined electronic and nuclear spin Hilbert space. If this trace is evaluated deterministically, by performing a separate time-dependent wavepacket calculation for each initial state in the Hilbert space, the computational effort scales as O(Z 2 log⁡Z), where Z is the total number of nuclear spin states. Here we show that the trace can also be evaluated stochastically, by exploiting the properties of spin coherent states. This results in a computational effort of O(MZlog⁡Z), where M is the number of Monte Carlo samples needed for convergence. Example calculations on a strongly coupled radical pair with Z>10 6 show that the singlet yield can be converged to graphical accuracy using just M=200 samples, resulting in a speed up by a factor of >5000 over a standard deterministic calculation. We expect that this factor will greatly facilitate future quantum mechanical simulations of a wide variety of radical pairs of interest in chemistry and biology.

Chiral gold(I vs chiral silver complexes as catalysts for the enantioselective synthesis of the second generation GSK-hepatitis C virus inhibitor

Directory of Open Access Journals (Sweden)

María Martín-Rodríguez

2011-07-01

Full Text Available The synthesis of a GSK 2nd generation inhibitor of the hepatitis C virus, by enantioselective 1,3-dipolar cycloaddition between a leucine derived iminoester and tert-butyl acrylate, was studied. The comparison between silver(I and gold(I catalysts in this reaction was established by working with chiral phosphoramidites or with chiral BINAP. The best reaction conditions were used for the total synthesis of the hepatitis C virus inhibitor by a four step procedure affording this product in 99% ee and in 63% overall yield. The origin of the enantioselectivity of the chiral gold(I catalyst was justified according to DFT calculations, the stabilizing coulombic interaction between the nitrogen atom of the thiazole moiety and one of the gold atoms being crucial.

THEORETICAL STUDY OF THE REMOVAL OF THE TROPOSPHERIC FORMALDEHYDE BY REACTION WITH OH* AND NH3* RADICALS

OpenAIRE

Cjuno H., Jesús A.; Arroyo C., Juan; Cubas C., Roger

2014-01-01

In the context of atmospheric chemistry, two reactions radical-molecule of hydrogen abstraction have been studied. These are the OH' and NO3: radical (oxidizers agents in the troposphere) with formaldehyde in gas-phase. The calculations were carried out using the PM3 and . Ab initio methods of the UHF type. The results have allowed us to estimate the corresponding times of tropospheric permanency and the implications of these reactions in the removal of forrnaldehyde and similars from the low...

Kinetics of the Reaction of CH3O2 Radicals with OH Studied over the 292-526 K Temperature Range.

Science.gov (United States)

Yan, Chao; Kocevska, Stefani; Krasnoperov, Lev N

2016-08-11

Reaction of methyl peroxy radicals with hydroxyl radicals, CH3O2 + OH → CH3O + HO2 (1a) and CH3O2 + OH → CH2OO + H2O (1b) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 292-526 K temperature range and pressure 1 bar (bath gas He). Hydroxyl radicals were generated in the reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N2O at 193.3 nm, with H2O. Methyl peroxy radicals were generated in the reaction of methyl radicals, CH3, produced in the photolysis of acetone at 193.3 nm, and subsequent reaction of CH3 with O2. Temporal profiles of OH were monitored via transient absorption of light from a DC discharge H2O/Ar low-pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light was determined by accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The overall rate constant of the reaction is k1a+1b = (8.4 ± 1.7) × 10(-11)(T/298 K)(-0.81) cm(3) molecule(-1) s(-1) (292-526 K). The branching ratio of channel 1b at 298 K is less than 5%.

Bibliographies on radiation chemistry: Pt. 12; Rate constants for reactions of nonmetallic inorganic radicals in aqueous solution

Energy Technology Data Exchange (ETDEWEB)

Helman, W P; Ross, A B [Notre Dame Univ., IN (USA). Radiation Chemistry Data Center

1990-01-01

Rate constants have been determined by pulse radiolysis, flash photolysis, and other methods, for a wide variety of reactions involving transient radicals in aqueous solution. Reliable rate constants have been established for reactions of radicals from water (e{sub aq}{sup -}, {center dot}H, {center dot}OH/{center dot}O{sup -}) and the data have been tabulated (Buxton, 1988) through 1986. Kinetic data for HO{sub 2}{center dot}/O{sub 2}{center dot}{sup -} were tabulated. (Bielski, 1985) from papers published through 1983. A compilation of rate constants, from the literature through Mid-1987, for other nonmetallic inorganic radicals has also appeared recently (Neta, 1988). Together, these compilations contain rate constants for more than 6,000 different reactions, reported in about 2,000 references. The present bibliography provides a list of relevant references which have been collected since the publication of the above-mentioned compilations. The list contains references received through the end of December, 1989. (author).

Graphene-based hybrid for enantioselective sensing applications.

Science.gov (United States)

Zor, Erhan; Morales-Narváez, Eden; Alpaydin, Sabri; Bingol, Haluk; Ersoz, Mustafa; Merkoçi, Arben

2017-01-15

Chirality is a major field of research of chemical biology and is essential in pharmacology. Accordingly, approaches for distinguishing between different chiral forms of a compound are of great interest. We report on an efficient and generic enantioselective sensor that is achieved by coupling reduced graphene oxide with γ-cyclodextrin (rGO/γ-CD). The enantioselective sensing capability of the resulting structure was operated in both electrical and optical mode for of tryptophan enantiomers (D-/L-Trp). In this sense, voltammetric and photoluminescence measurements were conducted and the experimental results were compared to molecular docking method. We gain insight into the occurring recognition mechanism with selectivity toward D- and L-Trp as shown in voltammetric, photoluminescence and molecular docking responses. As an enantioselective solid phase on an electrochemical transducer, thanks to the different dimensional interaction of enantiomers with hybrid material, a discrepancy occurs in the Gibbs free energy leading to a difference in oxidation peak potential as observed in electrochemical measurements. The optical sensing principle is based on the energy transfer phenomenon that occurs between photoexcited D-/L-Trp enantiomers and rGO/γ-CD giving rise to an enantioselective photoluminescence quenching due to the tendency of chiral enantiomers to form complexes with γ-CD in different molecular orientations as demonstrated by molecular docking studies. The approach, which is the first demonstration of applicability of molecular docking to show both enantioselective electrochemical and photoluminescence quenching capabilities of a graphene-related hybrid material, is truly new and may have broad interest in combination of experimental and computational methods for enantiosensing of chiral molecules. Copyright © 2016 Elsevier B.V. All rights reserved.

Research concerning ionic and free radical reactions in radiation chemistry. Progress report, September 15, 1976--September 15, 1977

International Nuclear Information System (INIS)

Williams, T.F.

1977-01-01

Progress is reported on ESR studies of fluorocarbon radicals and intermediate radicals. A detailed study was made of the dimethyl, diethyl, and di-n-propyl carbonates. Studies were continued on hydrogen-atom abstraction reactions at low temperatures with view to evaluating the contribution from quantum-mechanical tunneling. Detection of the transient dimer radical anion of acetonitrile in the upper crystalline phase at -50 0 C is reported. Abstracts of current reports are included on electron attachment to fluorocarbons hydrogen atom abstraction by methyl radicals. EPR spectra of the tetrafluoroethylene radical anion, and addition of tetrafluoroethylene to the tetrafluoroethylene radical anion

Enantioselective Intramolecular CH-Insertions upon Cu-Catalyzed Decomposition of Phenyliodonium Ylides

Directory of Open Access Journals (Sweden)

Christelle Boléa

2001-02-01

Full Text Available The Cu-catalyzed intramolecular CH insertion of phenyliodonium ylide 5b has been investigated at 0° C in the presence of several chiral ligands. Enantioselectivities vary in the range of 38–72 %, and are higher than those resulting from reaction of the diazo compound 5c at 65° C. The results are consistent with a carbenoid mechanism for Cu-catalyzed decomposition of phenyliodonium ylides.

Rhodium-catalyzed chemo-, regio-, and enantioselective addition of 2-pyridones to terminal allenes.

Science.gov (United States)

Li, Changkun; Kähny, Matthias; Breit, Bernhard

2014-12-08

A rhodium-catalyzed chemo-, regio-, and enantioselective addition of 2-pyridones to terminal allenes to give branched N-allyl 2-pyridones is reported. Preliminary mechanistic studies support the hypothesis that the reaction was initiated from the more acidic 2-hydroxypyridine form, and the initial kinetic O-allylation product was finally converted into the thermodynamically more stable N-allyl 2-pyridones. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Reaction mechanisms and kinetics of the iminovinylidene radical with NO: Ab initio study

Energy Technology Data Exchange (ETDEWEB)

Hsiao, Ming-Kai; Chung, Yi-Hua; Hung, Yu-Ming; Chen, Hui-Lung, E-mail: chl3@faculty.pccu.edu.tw [Department of Chemistry and Institute of Applied Chemistry, Chinese Culture University, Taipei 111, Taiwan (China)

2014-05-28

The nitric oxide (NO) is a notorious compound for polluting environment. Recent year, removing nitric oxide from the atmosphere becomes a focus of the investigation. In our work, we study the iminovinylidene (HNCC) radical reacted with NO molecule. The mechanism and kinetic for reaction of the HNCC radical with the NO molecule is investigated via considering the possible channels of the N and O atoms of NO attacking the N and C atoms of the HNCC based on the high level ab initio molecular orbital calculations in conjunction with variational TST and RRKM calculations. The species involved have been optimized at the B3LYP/6-311++G(3df,2p) level and their single-point energies are refined by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) method. The calculated potential energy surfaces indicated that energetically the most favorable channel for the HNCC + NO reaction was predicted to be the formation of HNC+CNO (P8) product via the addition reaction of the C atom of HNCC radical and the N atom of NO with the head to head orientation. To rationalize the scenario of the calculated results, we also employ the Fukui functions and HSAB theory to seek for a possible explanation. In addition, the reaction rate constants were calculated using VariFlex code, and the results show that the total rate coefficient, k{sub total}, at Ar pressure 760 Torr can be represented with an equation: k{sub total} = 6.433 × 10{sup −11} T {sup 0.100} exp(0.275 kcal mol{sup −1}/RT) at T = 298–3000 K, in units of cm{sup 3} molecule{sup −1} s{sup −1}.

Free radical transfer in polymers

International Nuclear Information System (INIS)

Sonntag, C. von; Bothe, E.; Ulanski, P.

1998-01-01

For the present study of free-radical transfer in polymers pulse radiolysis and product studies have been carried out in aqueous solutions using thus far only the water-soluble polymers polyacrylic acid, polymethacrylic acid and polyvinyl alcohol. When OH radicals, generated in the radiolysis of N 2 O-saturated aqueous solutions, react with polymers the lifetime of the polymer radical thus created very much depends on the number of radicals per polymer chain. When there are a large number of radicals per chain their bimolecular decay may be faster than the corresponding (diffusion controlled) decay of monomeric radicals, but when the macromolecule contains only few or even just one radical their lifetime is considerably prolonged. Highly charged polymers such as polyacrylic acid at high pH attain a rod-like conformation which again favors a long lifetime of the radicals. Under such conditions, radical transfer reactions can occur. For example, in polyacrylic acid OH radicals generate two kinds of radicals side by side. The radical in β-position to the carboxylate group converts into the thermodynamically more stable α-radicals by an H-transfer reaction as can be followed by spectrophotometry. Besides radical transfer reactions β-fragmentation reactions occur causing chain scission. Such reactions can be followed in a pulse radiolysis experiment by conductometry, because counter ions are released upon chain scission. Such a process is especially effective in the case of polymethacrylic acid, where it results in a chain depolymerization. An intramolecular H-abstraction is also observed in the γ-radiolysis of polyacrylic acid with the corresponding peroxyl radicals. This causes a chain reaction to occur. The resulting hydroperoxides are unstable and decarboxylate given rise to acetylacetone-like products. In polyvinyl alcohol the peroxyl radicals in α-position to the alcohol function undergo HO 2 -elimination. This prevents a scission of the polymer chain in the

Products of aqueous vitamin B5 (pantothenic acid) formed by free radical reactions

Energy Technology Data Exchange (ETDEWEB)

Schittl, H. [Section of Radiation Biology, Department of Nutritional Sciences, University of Vienna, Althanstr. 14, UZA II, A-1090 Vienna (Austria); Quint, R.M. [Section of Radiation Biology, Department of Nutritional Sciences, University of Vienna, Althanstr. 14, UZA II, A-1090 Vienna (Austria); Getoff, N. [Section of Radiation Biology, Department of Nutritional Sciences, University of Vienna, Althanstr. 14, UZA II, A-1090 Vienna (Austria)]. E-mail: nikola.getoff@univie.ac.at

2007-10-15

The radiolysis of aqueous vitamin B5 (pantothenic acid) has been investigated under various experimental conditions. The highest vitamin degradation (G=3.22) was observed in solutions saturated with N{sub 2}O, where 90% OH radicals are operating. As final products, the following were established: aldehydes, carboxylic acids and ammonia. Their yield strongly depends on the presence/absence of air as well as on N{sub 2}O (used to convert e{sub aq} {sup -} into OH) and was determined as a function of absorbed radiation dose. HPLC-analysis showed that in all media, a main product is formed, having the highest yield in aerated solutions. Based on the chemical analysis, it appears that the OH radicals are most involved in the degradation process. A precise sequence of the reaction steps could not be given presently, because of the implication of many simultaneous reactions.

Development of Environment-Friendly Insecticides Based on Enantioselectivity: Bifenthrin as a Case.

Science.gov (United States)

Qian, Yi; Zhou, Peixue; Zhang, Quan

2017-01-01

Chiral insecticides significantly contribute to the environmental pollutions recently. As the development of industry and agriculture, increasing number of chiral insecticides are to be introduced into the market. However, their enantioselective toxicology to ecosystem still remains uncertain. In this review, we embarked on a structured search of bibliographic databases for peer-reviewed articles regarding the enantioselective effects of bifenthrin, a typical chiral insecticide, on both target and non-target species. With this enantioselective property of chiral insecticides, they often exhibit adverse effects on non-target species enantioselectively. Specifically, the enantioselective effects of bifenthrin on target and non-target organisms were discussed. In target species, R-bifenthrin exerts more significant activities in deinsectization, compared with S-bifenthrin. On the other hand, Sbifenthrin is more toxic to non-target species than R-bifenthrin, which suggests that the application of sole enantiomer is more efficient and environment-friendly than that of racemate. This review confirms the choice of environment-friendly insecticides from the perspective of the enantioselectivity of chiral insecticides. To make insecticides more efficient to target species and less toxic to non-target species, further research should be done to investigated the potential effects of targetactive enantiomers on non-target organisms as well as the enantioselective fate of enantiomers in multiple environmental matrix.

Exploiting nanospace for asymmetric catalysis: confinement of immobilized, single-site chiral catalysts enhances enantioselectivity.

Science.gov (United States)

Thomas, John Meurig; Raja, Robert

2008-06-01

In the mid-1990s, it became possible to prepare high-area silicas having pore diameters controllably adjustable in the range ca. 20-200 Å. Moreover, the inner walls of these nanoporous solids could be functionalized to yield single-site, chiral, catalytically active organometallic centers, the precise structures of which could be determined using in situ X-ray absorption and FTIR and multinuclear magic angle spinning (MAS) NMR spectroscopy. This approach opened up the prospect of performing heterogeneous enantioselective conversions in a novel manner, under the spatial restrictions imposed by the nanocavities within which the reactions occur. In particular, it suggested an alternative method for preparing pharmaceutically and agrochemically useful asymmetric products by capitalizing on the notion, initially tentatively perceived, that spatial confinement of prochiral reactants (and transition states formed at the chiral active center) would provide an altogether new method of boosting the enantioselectivity of the anchored chiral catalyst. Initially, we anchored chiral single-site heterogeneous catalysts to nanopores covalently via a ligand attached to Pd(II) or Rh(I) centers. Later, we employed a more convenient and cheaper electrostatic method, relying in part on strong hydrogen bonding. This Account provides many examples of these processes, encompassing hydrogenations, oxidations, and aminations. Of particular note is the facile synthesis from methyl benzoylformate of methyl mandelate, which is a precursor in the synthesis of pemoline, a stimulant of the central nervous system; our procedure offers several viable methods for reducing ketocarboxylic acids. In addition to relying on earlier (synchrotron-based) in situ techniques for characterizing catalysts, we have constructed experimental procedures involving robotically controlled catalytic reactors that allow the kinetics of conversion and enantioselectivity to be monitored continually, and we have access to

To the reaction of silyl radicals. The ratio of disproportionation/recombination

International Nuclear Information System (INIS)

Reimann, B.; Matten, A.; Laupert, R.; Potzinger, P.

1977-01-01

Silyl radicals react in two distinct ways: (1) recombination to a vibrationally highly excited disilane, and (2) disproportionation to silylene and silane. Silylene disappears by a very fast insertion reaction in which disilane is formed. - Both reaction paths (1) and (2) can be distinguished by isotopic labelling. Disilane formed by (1), either undergoes unimolecular decomposition forming silylene and silane or it is stabilized through collisions. The ratio of disproportionation to recombination products is therefore pressure dependent. The disproportionation to recombination ratio, as calculated by extrapolation to infinite pressure, is 0.7 +- 0.1. - Photoionization mass spectrometry has been applied for the quantitative analysis of the deuterated disilanes. (orig.) [de

Rhodium/chiral diene-catalyzed asymmetric 1,4-addition of arylboronic acids to chromones: a highly enantioselective pathway for accessing chiral flavanones.

Science.gov (United States)

He, Qijie; So, Chau Ming; Bian, Zhaoxiang; Hayashi, Tamio; Wang, Jun

2015-03-01

Chromone has been noted to be one of the most challenging substrates in the asymmetric 1,4-addition of α,β-unsaturated carbonyl compounds. By employing the rhodium complex associated with a chiral diene ligand, (R,R)-Ph-bod*, the 1,4-addition of a variety of arylboronic acids was realized to give high yields of the corresponding flavanones with excellent enantioselectivities (≥97% ee, 99% ee for most substrates). Ring-opening side products, which would lead to erosion of product enantioselectivity, were not observed under the stated reaction conditions. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kinetics of Several Oxygen-Containing Carbon-Centered Free Radical Reactions with Nitric Oxide.

Science.gov (United States)

Rissanen, Matti P; Ihlenborg, Marvin; Pekkanen, Timo T; Timonen, Raimo S

2015-07-16

Kinetics of four carbon-centered, oxygen-containing free radical reactions with nitric oxide (NO) were investigated as a function of temperature at a few Torr pressure of helium, employing flow tube reactors coupled to a laser-photolysis/resonance-gas-discharge-lamp photoionization mass spectrometer (LP-RPIMS). Rate coefficients were directly determined from radical (R) decay signals under pseudo-first-order conditions ([R]0 ≪ [NO]). The obtained rate coefficients showed negative temperature dependences, typical for a radical-radical association process, and can be represented by the following parametrizations (all in units of cm(3) molecule(-1) s(-1)): k(CH2OH + NO) = (4.76 × 10(-21)) × (T/300 K)(15.92) × exp[50700/(RT)] (T = 266-363 K, p = 0.79-3.44 Torr); k(CH3CHOH + NO) = (1.27 × 10(-16)) × (T/300 K)(6.81) × exp[28700/(RT)] (T = 241-363 K, p = 0.52-3.43 Torr); k(CH3OCH2 + NO) = (3.58 ± 0.12) × 10(-12) × (T/300 K)(-3.17±0.14) (T = 221-363 K, p = 0.50-0.80 Torr); k(T)3 = 9.62 × 10(-11) × (T/300 K)(-5.99) × exp[-7100/(RT)] (T = 221-473 K, p = 1.41-2.95 Torr), with the uncertainties given as standard errors of the fits and the overall uncertainties estimated as ±20%. The rate of CH3OCH2 + NO reaction was measured in two density ranges due to its observed considerable pressure dependence, which was not found in the studied hydroxyalkyl reactions. In addition, the CH3CO + NO rate coefficient was determined at two temperatures resulting in k298K(CH3CO + NO) = (5.6 ± 2.8) × 10(-13) cm(3) molecule(-1) s(-1). No products were found during these experiments, reasons for which are briefly discussed.

Kinetics and mechanism of the reaction of recombination of vinyl and hydroxyl radicals

Science.gov (United States)

Knyazev, Vadim D.

2017-10-01

The recombination of the vinyl (C2H3) and the hydroxyl (OH) radicals was studied computationally using quantum chemistry and master equation/RRKM. The reaction mechanism includes the initial addition, several isomerization steps, and decomposition via seven different channels. The spectrum of products demonstrates temperature dependence in the 300-3000 K range. At low temperatures (below 1600 K), CH3 + HCO products are dominant but at elevated temperatures vinoxy radical (CH2CHO) and hydrogen atom become more important. The acetyl (CH3CO) + H products and formation of vinylidene (CH2C:) and water products are minor but non-negligible.

Theoretical description of spin-selective reactions of radical pairs diffusing in spherical 2D and 3D microreactors

International Nuclear Information System (INIS)

Ivanov, Konstantin L.; Lukzen, Nikita N.; Sadovsky, Vladimir M.

2015-01-01

In this work, we treat spin-selective recombination of a geminate radical pair (RP) in a spherical “microreactor,” i.e., of a RP confined in a micelle, vesicle, or liposome. We consider the microreactor model proposed earlier, in which one of the radicals is located at the center of the micelle and the other one undergoes three-dimensional diffusion inside the micelle. In addition, we suggest a two-dimensional model, in which one of the radicals is located at the “pole” of the sphere, while the other one diffuses on the spherical surface. For this model, we have obtained a general analytical expression for the RP recombination yield in terms of the free Green function of two-dimensional diffusion motion. In turn, this Green function is expressed via the Legendre functions and thus takes account of diffusion over a restricted spherical surface and its curvature. The obtained expression allows one to calculate the RP recombination efficiency at an arbitrary magnetic field strength. We performed a comparison of the two models taking the same geometric parameters (i.e., the microreactor radius and the closest approach distance of the radicals), chemical reactivity, magnetic interactions in the RP and diffusion coefficient. Significant difference between the predictions of the two models is found, which is thus originating solely from the dimensionality effect: for different dimensionality of space, the statistics of diffusional contacts of radicals becomes different altering the reaction yield. We have calculated the magnetic field dependence of the RP reaction yield and chemically induced dynamic nuclear polarization of the reaction products at different sizes of the microreactor, exchange interaction, and spin relaxation rates. Interestingly, due to the intricate interplay of diffusional contacts of reactants and spin dynamics, the dependence of the reaction yield on the microreactor radius is non-monotonous. Our results are of importance for (i) interpreting

Radical transfer between proteins: role of tyrosine, tryptophan and protein peroxyl radicals

International Nuclear Information System (INIS)

Irwin, J.A.; Ostdal, H.; Davies, M.J.

1998-01-01

Reaction of the Fe(III) forms of the heme proteins myoglobin (Mb) and horseradish peroxidase (HRP) with H 2 O 2 gives rise to high-oxidation-state heme-derived species which can be described as a Fe(IV)-oxo porphyrin radical-cation ('Compound 1'). In the case of Mb, the Fe(IV)-oxo porphyrin radical-cation undergoes rapid electron transfer with the surrounding protein to give protein (globin)-derived radicals and an Fe(lV)-oxo species ('Compound 2'). The globin-derived radicals have been shown to be located at two (or more) sites: Tyr-103 or Trp-14, with the latter radical known to react with oxygen to give a Trp-derived peroxyl radical (Mb-Trp-OO*). With HRP, the Fe(lV)-oxo porphyrin radical-cation carries out two successive one-electron oxidation reactions at the exposed heme edge to give firstly 'Compound 2' [the Fe(lV)oxo species] and then the resting Fe(III) state of the enzyme. n this study we have investigated whether the Trp-14 peroxyl radical from Mb and the Compound 1 and 2 species from HRP (in the absence and presence of free Tyr) can oxidise amino acids, peptides and proteins. Such reactions constitute intermolecular protein-to-protein radical transfer reactions and hence protein chain-oxidation. We have also examined whether these oxidants react with antioxidants. Reaction of these heme-protein derived oxidants with amino acids, proteins and antioxidants has been carried out at room temperature for defined periods of time before freeze-quenching to 77K to halt reaction. The radical species present in the reaction system at the time of freezing were subsequently examined by EPR spectroscopy at 77K. Three free amino acids, Tyr, Trp and Cys (with Cys the least efficient) have been shown to react rapidly with Mb-Trp-OO*, as evidenced by the loss of the characteristic EPR features of Mb-Trp-OO* on inclusion of increasing concentrations of the amino acids. All other amino acids are much less reactive. Evidence has also been obtained for (inefficient) hydrogen

Enantioselective synthesis of α-oxy amides via Umpolung amide synthesis.

Science.gov (United States)

Leighty, Matthew W; Shen, Bo; Johnston, Jeffrey N

2012-09-19

α-Oxy amides are prepared through enantioselective synthesis using a sequence beginning with a Henry addition of bromonitromethane to aldehydes and finishing with Umpolung Amide Synthesis (UmAS). Key to high enantioselection is the finding that ortho-iodo benzoic acid salts of the chiral copper(II) bis(oxazoline) catalyst deliver both diastereomers of the Henry adduct with high enantiomeric excess, homochiral at the oxygen-bearing carbon. Overall, this approach to α-oxy amides provides an innovative complement to alternatives that focus almost entirely on the enantioselective synthesis of α-oxy carboxylic acids.

tri-n-butyltin hydride-mediated radical reaction of a 2-iodobenzamide: formation of an unexpected carbon-tin bond

International Nuclear Information System (INIS)

Oliveira, Marcelo T.; Alves, Rosemeire B.; Cesar, Amary; Prado, Maria Auxiliadora F.; Alves, Ricardo J.; Queiroga, Carla G.; Santos, Leonardo S.; Eberlin, Marcos N.

2007-01-01

The tri-n-butyltin hydride-mediated reaction of methyl 2,3-di-O-benzyl-4-O-trans-cinnamyl- 6-deoxy-6-(2-iodobenzoylamino)-α-D-galactopyranoside afforded an unexpected aryltributyltin compound. The structure of this new tetraorganotin(IV) product has been elucidated by 1 H, 13 C NMR spectroscopy, COSY and HMQC experiments and electrospray ionization mass spectrometry (ESI-MS). The formation of this new compound via a radical coupling reaction and a radical addition-elimination process is discussed. (author)

ESR investigation of the reactions of glutathione, cysteine and penicillamine thiyl radicals: competitive formation of RSOcenter dot, Rcenter dot, RSSRcenter dot-. , and RSScenter dot

Energy Technology Data Exchange (ETDEWEB)

Becker, David; Swarts, Steven; Champagne, Mark; Sevilla, M D

1988-05-01

The reactions of cysteine, glutathione and penicillamine thiyl radicals with oxygen and their parent thiols in frozen solutions have been elucidated with e.s.r. The major sulfur radicals observed are: (1) thiyl radicals, RS center dot; (2) disulfide radical anions, RSSR anion radicals; (3) perthiyl radicals, RSS center dot and upon introduction of oxygen; (4) sulfinyl radicals, RSO center dot, where R represents the remainder of the cysteine, glutathione or penicillamine moiety. The radical product observed depends on pH, concentration of thiol, and presence or absence of molecular oxygen. The sulfinyl radical is a ubiquitous intermediate, peroxyl radical attack on thiols may lead to sulfinyl radicals. The authors elaborate the observed reaction sequences that lead to sulfinyl radicals and, using /sup 17/O isotopic substitution studies, demonstrate the oxygen atom in sulfinyl radicals originates from dissolved molecular oxygen. The glutathione radical is found to abstract hydrogen from the ..cap alpha..-carbon position on the cysteine residue of glutathione to form a carbon-centred radical.

Application of a Heterogeneous Chiral Titanium Catalyst Derived from Silica-Supported 3-Aryl H8-BINOL to Enantioselective Alkylation and Arylation of Aldehydes.

Science.gov (United States)

Akai, Junichiro; Watanabe, Satoshi; Michikawa, Kumiko; Harada, Toshiro

2017-07-07

A 3-aryl H 8 -BINOL was grafted on the surface of silica gel using a hydrosilane derivative as a precursor, and the resulting silica-supported ligand (6 mol %) was employed in the enantioselective alkylation and arylation of aldehydes in the presence of Ti(O i Pr) 4 . The reactions using Et 2 Zn, Et 3 B, and aryl Grignard reagents all afforded the corresponding adducts in high enantioselectivities and yields. The silica-immobilized titanium catalyst could be reused up to 14 times without appreciable deterioration of the activity.

Copper(II)-catalyzed exo and enantioselective cycloadditions of azomethine imines.

Science.gov (United States)

Sibi, Mukund P; Rane, Digamber; Stanley, Levi M; Soeta, Takahiro

2008-07-17

A strategy for exo and enantioselective 1,3-dipolar cycloaddition of azomethine imines to 2-acryloyl-3-pyrazolidinone is described. The corresponding cycloadducts are isolated with high diastereoselectivities (up to >96:4 exo/endo) and enantioselectivities (up to 98% ee).

Reaction studies of hot silicon and germanium radicals. Period covered: September 1, 1977--August 31, 1978

International Nuclear Information System (INIS)

Gaspar, P.P.

1978-01-01

The experimental approach to attaining the goals of this research program is briefly outlined and the progress made in the last year is reviewed in sections entitled: primary steps in the reaction of recoiling silicon and germanium atoms and the identification of reactive intermediates; thermally induced silylene and germylene reactions; the role of ionic reactions in the chemistry of recoiling silicon atoms and other ion-molecule reactions studies; and silicon free radical chemistry

A shock tube study of the reactions of the hydroxyl radical with combustion species

Energy Technology Data Exchange (ETDEWEB)

Cohen, N.; Koffend, J.B. [The Aerospace Corporation, Los Angeles, CA (United States)

1993-12-01

To extend the semi-empirical techniques of Benson and coworkers, and to extend the database of reliable high temperature measurements of OH radicals with hydrocarbons and other fuels and their decomposition products, the authors undertook a research program with both experimental and computational tasks. The experimental goal was to design a procedure for measuring, at combustion temperatures, the reaction rate coefficients of OH radicals with fuels and other species of importance in combustion or propulsion systems. The computational effort was intended to refine the semi-empirical transition-state-theory procedures for extrapolating rate coefficients of reactions of OH with combustion species of interest, for predicting rate coefficients for species not studied in the laboratory, and to examine the ability of the theory to predict rate coefficients for different pathways in the case the reagent possessed more than one nonequivalent H atoms.

Reaction of Pentanol isomers with OH radical – A theoretical perspective

Science.gov (United States)

Aazaad, Basheer; Lakshmipathi, Senthilkumar

2018-05-01

The stability of all the three isomeric forms of Pentanol has been examined with relative energy analysis. Even though 2-Pentanol is predicted to be most stable isomeric form, all the three isomeric forms undergo hydrogen atom abstraction reaction with OH radical. Among the proposed 18 different hydrogen atom abstraction reaction, the abstraction from CH2 and CH functional group is found to be a favourable reactive site with low energy barrier in M06-2X/6-311+G(d,p) level of theory. Wiberg bond order analysis shows all the abstraction reactions are concreted but not synchronic in nature. Using force analysis, the calculated work done of individual reaction regions illustrates that structural rearrangements drive the reaction with higher contribution to the energy barrier. The rate constant calculated at M06-2X method for the most favourable reaction is well matched with available experimental data. Using the reported atmospheric OH concentration (1 × 106 molecules/cm3), the life time of 1-Pentanol, 2-Pentanol and 3-Pentanol has calculated to be 18.66, 0.36 and 2.86 days, respectively.

Enantioselective biotransformation of propranolol to the active metabolite 4-hydroxypropranolol by endophytic fungi

Directory of Open Access Journals (Sweden)

Keyller Bastos Borges

2011-01-01

Full Text Available The enantioselective biotransformation of propranolol (Prop by the endophytic fungi Phomopsis sp., Glomerella cingulata, Penicillium crustosum, Chaetomium globosum and Aspergillus fumigatus was investigated by studying the kinetics of the aromatic hydroxylation reaction with the formation of 4-hydroxypropranolol (4-OH-Prop. Both Prop enantiomers were consumed by the fungi in the biotransformation process, but the 4-hydroxylation reaction yielded preferentially (--(S-4-OH-Prop. The quantity of metabolites biosynthesized varied slightly among the evaluated endophytic fungi. These results show that all investigated endophytic fungi could be used as biosynthetic tools in biotransformation processes to obtain the enantiomers of 4-OH-Prop.

Metal-catalyzed living radical polymerization and radical polyaddition for precision polymer synthesis

Energy Technology Data Exchange (ETDEWEB)

Mizutani, M; Satoh, K [Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603 (Japan); Kamigaito, M, E-mail: kamigait@apchem.nagoya-u.ac.j

2009-08-01

The metal-catalyzed radical addition reaction can be evolved into two different polymerization mechanisms, i.e.; chain- and step-growth polymerizations, while both the polymerizations are based on the same metal-catalyzed radical formation reaction. The former is a widely employed metal-catalyzed living radical polymerization or atom transfer radical polymerization of common vinyl monomers, and the latter is a novel metal-catalyzed radical polyaddition of designed monomer with an unconjugated C=C double bond and a reactive C-Cl bond in one molecule. The simultaneous ruthenium-catalyzed living radical polymerization of methyl acrylate and radical polyaddition of 3-butenyl 2-chloropropionate was achieved with Ru(Cp*)Cl(PPh{sub 3}){sub 2} to afford the controlled polymers, in which the homopolymer segments with the controlled chain length were connected by the ester linkage.

UV-visible spectrum of the phenyl radical and kinetics of its reaction with NO in the gas phase

DEFF Research Database (Denmark)

Wallington, T.J.; Egsgaard, H.; Nielsen, O.J.

1998-01-01

Pulse radiolysis transient UV-visible absorption spectroscopy was used to study the UV-visible absorption spectrum (225-575 nm) of the phenyl radical, C6H5(.), and kinetics of its reaction with NO. Phenyl radicals have a strong broad featureless absorption in the region of 225-340 nm...

Pathways for the reaction of the butadiene radical cation, [C{sub 4}H{sub 6}]{sup {sm{underscore}bullet}+}, with ethylene

Energy Technology Data Exchange (ETDEWEB)

Hofmann, M.; Schaefer, H.F. III

1999-11-04

The Diels-Alder (DA) reaction, a [4+2] cycloaddition used to build six membered rings, is one of the most valuable cycloadditions in organic chemistry. In cases where the ene does not add to the diene (even with the help of Lewis acids which may reduce the electron density of one reactant by complexation) one electron oxidation (by an oxidizing agent or by photoinduced electron transfer (PET)) may accelerate the reaction. Reaction pathways for the addition of ethylene, 1, to butadiene radical cation, 2, involving H-shifts have been investigated at the coupled cluster UCCSD(T)/DZP//UMP2(fc)/DZP + ZPE level of theory. Activation energies are relatively low for [1,2]- (10.0 kcal mol{sup {minus}1}, TS-4/20) and [1,5]-hydrogen shifts (7.7 kcal mol{sup {minus}1}, TS-4/26) but are relatively high for [1,4]-(33.8 kcal mol{sup {minus}1}, TS-4/14) and [1,3]-H shifts (e.g., 42.2 kcal mol{sup {minus}1}, TS-12/13; 57.2 kcal mol{sup {minus}1}, TS-16/21). Several rearrangement reactions have been found to occur below the energy limit of separated 1 + 2. The cyclopentenyl cation, [C{sub 5}{sub 7}]{sup +}, 18, experimentally observed as reaction product of the butadiene radical cation, 2, and ethylene, 1, in the gas phase may origin from various reaction pathways. The following reaction sequence has been identified as the lowest in energy path from 1 + 2 to 18 with all relative energies ({Delta}E{degree}) of transition structures below that of 1 + 2: (a) ethylene adds to the butadiene radical cation to form an open-chain distonic intermediate, that undergoes a [1,5]-H shift to the 1,4-hexadiene radical cation; (b) intramolecular [2+1] cycloaddition to methyl-cyclopenta-1,3-diyl intermediates, which can interconvert through a bicyclo[2.1.0]pentane radical cation; (c) [1,2]-H shift to the 3-methyl cyclopentene radical cation; (d) methyl radical loss to give cyclopenten-3-yl cation. Along this reaction pathway, {Delta}H{sup 298} is below that of 1 + 2; max. ({Delta}G{sup 298} by

Highly selective transformation of ammonia nitrogen to N2 based on a novel solar-driven photoelectrocatalytic-chlorine radical reactions system.

Science.gov (United States)

Ji, Youzhi; Bai, Jing; Li, Jinhua; Luo, Tao; Qiao, Li; Zeng, Qingyi; Zhou, Baoxue

2017-11-15

A highly selective method for transforming ammonia nitrogen to N 2 was proposed, based on a novel solar-driven photoelectrocatalytic-chlorine radical reactions (PEC-chlorine) system. The PEC-chlorine system was facilitated by a visible light response WO 3 nanoplate array (NPA) electrode in an ammonia solution containing chloride ions (Cl - ). Under illumination, photoholes from WO 3 promote the oxidation of Cl - to chlorine radical (Cl). This radical can selectively transform ammonia nitrogen to N 2 (79.9%) and NO 3 - (19.2%), similar to the breakpoint chlorination reaction. The ammonia nitrogen removal efficiency increased from 10.6% (PEC without Cl - ) to 99.9% with the PEC-chlorine system within 90 min operation, which can be attributed to the cyclic reactions between Cl - /Cl and the reaction intermediates (NH 2 , NHCl, etc.) that expand the degradation reactions from the surface of the electrodes to the whole solution system. Moreover, Cl is the main radical species contributing to the transformation of ammonia nitrogen to N 2 , which is confirmed by the tBuOH capture experiment. Compared to conventional breakpoint chlorination, the PEC-chlorine system is a more economical and efficient means for ammonia nitrogen degradation because of the fast removal rate, no additional chlorine cost, and its use of clean energy (since it is solar-driven). Copyright © 2017 Elsevier Ltd. All rights reserved.

Reaction studies of hot silicon and germanium radicals. Progress report, September 1, 1978-August 31, 1979

International Nuclear Information System (INIS)

Gaspar, P.P.

1979-01-01

The experimental approach to attaining the goals of this research program is briefly outlined and the progress made in the last year is reviewed in sections entitled: (a) Primary steps in the reaction of recoiling silicon and germanium atoms and the identification of reactive intermediates; (b) Thermally induced silylene and germylene reactions; (c) Silicon free radical chemistry; (d) The role of ionic reactions in the chemistry of recoiling silicon atoms

Verification of Radicals Formation in Ethanol-Water Mixture Based Solution Plasma and Their Relation to the Rate of Reaction.

Science.gov (United States)

Sudare, Tomohito; Ueno, Tomonaga; Watthanaphanit, Anyarat; Saito, Nagahiro

2015-12-03

Our previous research demonstrated that using ethanol-water mixture as a liquid medium for the synthesis of gold nanoparticles by the solution plasma process (SPP) could lead to an increment of the reaction rate of ∼35.2 times faster than that in pure water. This drastic change was observed when a small amount of ethanol, that is, at an ethanol mole fraction (χethanol) of 0.089, was added in the system. After this composition, the reaction rate decreased continuously. To better understand what happens in the ethanol-water mixture-based SPP, in this study, effect of the ethanol content on the radical formation in the system was verified. We focused on detecting the magnetic resonance of electronic spins using electron spin resonance spectroscopy to determine the type and quantity of the generated radicals at each χethanol. Results indicated that ethanol radicals were generated in the ethanol-water mixtures and exhibited maximum quantity at the xethanol of 0.089. Relationship between the ethanol radical yield and the rate of reaction, along with possible mechanism responsible for the observed phenomenon, is discussed in this paper.

Experimental and theoretical studies on gas-phase reactions of NO3 radicals with three methoxyphenols: Guaiacol, creosol, and syringol

Science.gov (United States)

Yang, Bo; Zhang, Haixu; Wang, Youfeng; Zhang, Peng; Shu, Jinian; Sun, Wanqi; Ma, Pengkun

2016-01-01

Methoxyphenols, lignin pyrolysis products, are major biomass combustion components and are considered potential tracers for wood smoke emissions. Their atmospheric reactivity, however, has not been well characterized. Guaiacol, creosol, and syringol are three typical methoxyphenols generated in relatively high concentrations in fresh wood smoke. In this study, the gas-phase reactions of NO3 radicals with these methoxyphenols were investigated using a laboratory-built vacuum ultraviolet photoionization gas time-of-flight mass spectrometer (VUV-GTOFMS) and off-line GC-MS. By combining experimental and theoretical methods, 4-nitroguaiacol, 6-nitroguaiacol, and 4,6-dinitroguaiacol were determined as the primary degradation products for guaiacol; similarly, 6-nitrocreosol and 3-nitrosyringol were identified for creosol and syringol, respectively. Using the relative rate method, rate constants at 298 K and 1 atm for the gas-phase reactions of guaiacol, creosol, and syringol with NO3 radicals were measured to be 3.2 × 10-12, 2.4 × 10-13, and 4.0 × 10-13 cm3 molecule-1 s-1, respectively. At a typical tropospheric concentration of NO3 radicals (5 × 108 molecule cm-3), atmospheric lifetimes for guaiacol, creosol, and syringol toward NO3 radicals were 0.2, 2.3, and 1.4 h, respectively. These results indicate that the reaction with NO3 radicals can be a major sink for methoxyphenols at night.

Nickel-catalyzed regio- and enantioselective aminolysis of 3,4-epoxy alcohols.

Science.gov (United States)

Wang, Chuan; Yamamoto, Hisashi

2015-04-08

The first catalytic regio- and enantioselective aminolysis of 3,4-epoxy alcohols has been accomplished. Under the catalysis of Ni(ClO4)2·6H2O, the C4 selective ring opening of various 3,4-epoxy alcohols proceeded in a stereospecific manner with high regioselectivities. Furthermore, with the Ni-BINAM catalytic system the enantioselective ring opening of 3,4-epoxy alcohols furnished various γ-hydroxy-δ-amino alcohols as products with complete regiocontrol and high enantioselectivities (up to 94% ee).

N,2,3,4-Tetrasubstituted Pyrrolidines through Tandem Lithium Amide Conjugate Addition/Radical Cyclization/Oxygenation Reactions

Czech Academy of Sciences Publication Activity Database

Kafka, František; Pohl, Radek; Císařová, I.; Mackman, R.; Bahador, G.; Jahn, Ullrich

2016-01-01

Roč. 2016, č. 22 (2016), s. 3862-3871 ISSN 1434-193X R&D Projects: GA ČR GA13-40188S Grant - others:COST(XE) CM1201 Institutional support: RVO:61388963 Keywords : tandem reactions * nitrogen heterocycles * Michael addition * radical reactions * cyclization * enolates Subject RIV: CC - Organic Chemistry Impact factor: 2.834, year: 2016

Pulse radiolysis study of reactions of alkyl and alkylperoxy radicals originating from methyl tert-butyl ether in the gas phase

DEFF Research Database (Denmark)

Langer, S.; Ljungström, E.; Ellermann, T.

1995-01-01

UV spectra and kinetics for the reactions of alkyl and alkylperoxy radicals from methyl tert-butyl ether (MTBE) were studied in 1 atm of SF6 by the pulse radiolysis-UV absorption technique. UV spectra for the radical mixtures were quantified from 215 to 340 nm. At 240 nm, sigma(R) = (2.6 +/- 0.4) X...... and the alkylperoxy radicals with NO and NO2 are (9.1 +/- 1.5) X 10(-13), (4.3 +/- 1.6) X 10(-12) and (1.2 +/- 0.3) X 10(-11) cm(3) molecule(-1) s(-1), respectively. The rate constants given above refer to reaction at the tert-butyl side of the molecule....

Linear free energy relationships between aqueous phase hydroxyl radical reaction rate constants and free energy of activation.

Science.gov (United States)

Minakata, Daisuke; Crittenden, John

2011-04-15

The hydroxyl radical (HO(•)) is a strong oxidant that reacts with electron-rich sites on organic compounds and initiates complex radical chain reactions in aqueous phase advanced oxidation processes (AOPs). Computer based kinetic modeling requires a reaction pathway generator and predictions of associated reaction rate constants. Previously, we reported a reaction pathway generator that can enumerate the most important elementary reactions for aliphatic compounds. For the reaction rate constant predictor, we develop linear free energy relationships (LFERs) between aqueous phase literature-reported HO(•) reaction rate constants and theoretically calculated free energies of activation for H-atom abstraction from a C-H bond and HO(•) addition to alkenes. The theoretical method uses ab initio quantum mechanical calculations, Gaussian 1-3, for gas phase reactions and a solvation method, COSMO-RS theory, to estimate the impact of water. Theoretically calculated free energies of activation are found to be within approximately ±3 kcal/mol of experimental values. Considering errors that arise from quantum mechanical calculations and experiments, this should be within the acceptable errors. The established LFERs are used to predict the HO(•) reaction rate constants within a factor of 5 from the experimental values. This approach may be applied to other reaction mechanisms to establish a library of rate constant predictions for kinetic modeling of AOPs.

Generation and propagation of radical reactions on proteins

DEFF Research Database (Denmark)

Hawkins, C L; Davies, Michael Jonathan

2001-01-01

The oxidation of proteins by free radicals is thought to play a major role in many oxidative processes within cells and is implicated in a number of human diseases as well as ageing. This review summarises information on the formation of radicals on peptides and proteins and how radical damage may...... be propagated and transferred within protein structures. The emphasis of this article is primarily on the deleterious actions of radicals generated on proteins, and their mechanisms of action, rather than on enzymatic systems where radicals are deliberately formed as transient intermediates. The final section...

DNA damage and radical reactions: Mechanistic aspects, formation in cells and repair studies

International Nuclear Information System (INIS)

Cadet, J.; Ravanat, J.L.; Carell, T.; Cellai, L.; Chatgilialoglu, Ch.; Gimisis, Th.; Miranda, M.; O'Neill, P.; Robert, M.

2008-01-01

Several examples of oxidative and reductive reactions of DNA components that lead to single and tandem modifications are discussed in this review. These include nucleophilic addition reactions of the one-electron oxidation-mediated guanine radical cation and the one-electron reduced intermediate of 8-bromo-purine 2'-de-oxy-ribo-nucleosides that give rise to either an oxidizing guanine radical or related 5',8-cyclo-purine nucleosides. In addition, mechanistic insights into the reductive pathways involved in the photolyase induced reversal of cyclo-buta-cli-pyrimidine and pyrimidine (6-4) pyrimidone photoproducts are provided. Evidence for the occurrence and validation in cellular DNA of (OH) · radical degradation pathways of guanine that have been established in model systems has been gained from the accurate measurement of degradation products. Relevant information on biochemical aspects of the repair of single and clustered oxidatively generated damage to DNA has been gained from detailed investigations that rely on the synthesis of suitable modified probes. Thus the preparation of stable carbocyclic derivatives of purine nucleoside containing defined sequence oligonucleotides has allowed detailed crystallographic studies of the recognition step of the base damage by enzymes implicated in the base excision repair (BER) pathway. Detailed insights are provided on the BER processing of non-double strand break bi-stranded clustered damage that may consist of base lesions, a single strand break or abasic sites and represent one of the main deleterious classes of radiation-induced DNA damage. (authors)

Free Radical Chemistry of Disinfection Byproducts 1: Kinetics of Hydrated Electron and Hydroxyl Radical Reactions with Halonitromethanes in Water

International Nuclear Information System (INIS)

B. J. Mincher; R. V. Fox; S. P. Mezyk; T. Helgeson; S. K. Cole; W. J. Cooper; P. R. Gardinali

2006-01-01

Halonitromethanes are disinfection-byproducts formed during ozonation and chlorine/chloramine treatment of waters that contain bromide ion and natural organic matter. In this study, the chemical kinetics of the free-radical-induced degradations of a series of halonitromethanes were determined. Absolute rate constants for hydroxyl radical, OH, and hydrated electron, e aq - , reaction with both chlorinated and brominated halonitromethanes were measured using the techniques of electron pulse radiolysis and transient absorption spectroscopy. The bimolecular rate constants obtained, k (M -1 s -1 ), for e aq - /OH, respectively, were the following: chloronitromethane (3.01 ± 0.40) x 10 10 /(1.94 ± 0.32) x 10 8 ; dichloronitromethane (3.21 ± 0.17) x 10 10 /(5.12 ± 0.77) x 10 8 ; bromonitromethane (3.13 ± 0.06) x 10 10 /(8.36 ± 0.57) x 107; dibromonitromethane (3.07 ± 0.40) x 10 10 /(4.75 ± 0.98) x 10 8 ; tribromonitromethane (2.29 ± 0.39) x 10 10 /(3.25 ± 0.67) x 10 8 ; bromochloronitromethane (2.93 ± 0.47) x 10 10 /(4.2 ± 1.1) x 10 8 ; bromodichloronitromethane (2.68 ± 0.13) x 10 10 /(1.02 ± 0.15) x 10 8 ; and dibromochloronitromethane (2.95 ± 0.43) x 10 10 /(1.80 ± 0.31) x 10 8 at room temperature and pH ∼7. Comparison data were also obtained for hydroxyl radical reaction with bromoform (1.50 ± 0.05) x 10 8 , bromodichloromethane (7.11 ± 0.26) x 10 7 , and chlorodibromomethane (8.31 ± 0.25) x 10 7 M -1 s -1 , respectively. These rate constants are compared to recently obtained data for trichloronitromethane and bromonitromethane, as well as to other established literature data for analogous compounds

Hexafluorobenzene: A powerful solvent for a noncovalent stereoselective organocatalytic Michael addition reaction

KAUST Repository

Lattanzi, Alessandra; De Fusco, Claudia; Russo, Alessio; Poater, Albert; Cavallo, Luigi

2012-01-01

A dramatic enhancement of the diastereo- and enantioselectivity in the nitro-Michael addition reaction organocatalysed by a commercially available α,α-l-diaryl prolinol was disclosed when performing the reaction in unconventional hexafluorobenzene

Integrative assessment of enantioselectivity in endocrine disruption and immunotoxicity of synthetic pyrethroids

Energy Technology Data Exchange (ETDEWEB)

Zhao Meirong [Research Center of Environmental Science, College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032 (China); Chen Fang [College of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310018 (China); Wang Cui; Zhang Quan [Research Center of Environmental Science, College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032 (China); Gan Jianying [Department of Environmental Sciences, University of California, Riverside, CA 92521 (United States); Liu Weiping, E-mail: wliu@zjut.edu.c [Research Center of Environmental Science, College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032 (China)

2010-05-15

The increasing release of chiral chemicals into the environment dictates attention to a better understanding of enantioselectivity in their human and ecotoxicological effects. Although enantioselectivity has been considered in many recent studies, there is little effort for discerning the connection between different processes, and as such, our current knowledge about chiral contaminants is rather scattered and incoherent. In this study, we simultaneously evaluated enantioselectivity of two chiral pesticides, lambda-cyhalothrin (LCT) and (Z)-cis-bifenthrin (cis-BF), in immunotoxicity to macrophage cells (RAW264.7), and endocrine disruption activity in human breast carcinoma cell line MCF-7. Analysis of cell proliferation, cell viability, apoptosis, and receptor gene expression showed significant differences between the enantiomers of LCT or cis-BF in estrogenic potential and immunocytotoxicity. The selectivity in these effects consistently followed the same direction, with (-)-LCT or 1S-cis-BF displaying a greater activity than its counterpart. The consistency was attributed to interplaying mechanisms in the closely interacting immune and endocrine systems. The underlying interplays suggest that other chiral xenobiotics may also show a directional enantioselectivity in immunotoxicity and endocrine toxicity. Given that many biological processes are inter-related, enantioselectivity may follow specific patterns that can be revealed via integrative assessments as demonstrated in this study. - Chiral contaminants should consider multiple effects and relate directions of enantioselectivity to their interplaying processes.

Integrative assessment of enantioselectivity in endocrine disruption and immunotoxicity of synthetic pyrethroids

International Nuclear Information System (INIS)

Zhao Meirong; Chen Fang; Wang Cui; Zhang Quan; Gan Jianying; Liu Weiping

2010-01-01

The increasing release of chiral chemicals into the environment dictates attention to a better understanding of enantioselectivity in their human and ecotoxicological effects. Although enantioselectivity has been considered in many recent studies, there is little effort for discerning the connection between different processes, and as such, our current knowledge about chiral contaminants is rather scattered and incoherent. In this study, we simultaneously evaluated enantioselectivity of two chiral pesticides, lambda-cyhalothrin (LCT) and (Z)-cis-bifenthrin (cis-BF), in immunotoxicity to macrophage cells (RAW264.7), and endocrine disruption activity in human breast carcinoma cell line MCF-7. Analysis of cell proliferation, cell viability, apoptosis, and receptor gene expression showed significant differences between the enantiomers of LCT or cis-BF in estrogenic potential and immunocytotoxicity. The selectivity in these effects consistently followed the same direction, with (-)-LCT or 1S-cis-BF displaying a greater activity than its counterpart. The consistency was attributed to interplaying mechanisms in the closely interacting immune and endocrine systems. The underlying interplays suggest that other chiral xenobiotics may also show a directional enantioselectivity in immunotoxicity and endocrine toxicity. Given that many biological processes are inter-related, enantioselectivity may follow specific patterns that can be revealed via integrative assessments as demonstrated in this study. - Chiral contaminants should consider multiple effects and relate directions of enantioselectivity to their interplaying processes.

Production of sulfate radical and hydroxyl radical by reaction of ozone with peroxymonosulfate: a novel advanced oxidation process.

Science.gov (United States)

Yang, Yi; Jiang, Jin; Lu, Xinglin; Ma, Jun; Liu, Yongze

2015-06-16

In this work, simultaneous generation of hydroxyl radical (•OH) and sulfate radical (SO4•−) by the reaction of ozone (O3) with peroxymonosulfate (PMS; HSO5−) has been proposed and experimentally verified. We demonstrate that the reaction between the anion of PMS (i.e.,SO52−) and O3 is primarily responsible for driving O3 consumption with a measured second order rate constant of (2.12 ± 0.03) × 10(4) M(-1) s(-1). The formation of both •OH and SO4•− from the reaction between SO52− and O3 is confirmed by chemical probes (i.e., nitrobenzene for •OH and atrazine forb oth •OH and SO4•−). The yields of •OH and SO4•− are determined to be 0.43 ± 0.1 and 0.45 ± 0.1 per mol of O3 consumption, respectively. An adduct,−O3SOO− + O3 → −O3SO5−, is assumed as the first step, which further decomposes into SO5•− and O3•−. The subsequent reaction of SO5•− with O3is proposed to generate SO4•−, while O3•− converts to •OH. A definition of R(ct,•OH) and R(ct,SO4•−) (i.e., respective ratios of •OH and SO4•− exposures to O3 exposure) is adopted to quantify relative contributions of •OH and SO4•−. Increasing pH leads to increases in both values of R(ct,•OH) and R(ct,SO4•−) but does not significantly affect the ratio of R(ct,SO4•−) to R(ct,•OH) (i.e., R(ct,SO4•−)/R(ct,•OH)), which represents the relative formation of SO4•− to •OH. The presence of bicarbonate appreciably inhibits the degradation of probes and fairly decreases the relative contribution of •OH for their degradation, which may be attributed to the conversion of both •OH and SO4•− to the more selective carbonate radical (CO3•−).Humic acid promotes O3 consumption to generate •OH and thus leads to an increase in the R(ct,•OH) value in the O3/PMS process,w hile humic acid has negligible influence on the R(ct,SO4•−) value. This discrepancy is reasonably explained by the negligible effect of humic acid on SO

CONCERNING CHAIN GROWTH SPECIFIC REACTION RATE AS A PART OF THE PROCESS OF METHYL METHACRYLATE MASS RADICAL POLYMERIZATION

Directory of Open Access Journals (Sweden)

A. A. Sultanova

2017-02-01

Full Text Available It is the chain growth specific reaction rate that was determined for the process of methyl methacrylate mass radical polymerization within the temperature range of 40–900 С in quasi-steady approximation by means of Monte Carlo method. The theoretical model of radical polymerization was developed taking the gel effect into account. Computer software was developed that enables to imitate radical polymerization process taking gel effect into account within the minimum run time. The programme was tested on asymptotic examples as well as was applied for methyl methacrylate mass radical polymerization. The programme makes it possible to calculate monomer conversion, molecular mass variation, molecular-mass distribution, etc.

Enantioselective epoxidation with chiral MN(III)(salen) catalysts: kinetic resolution of aryl-substituted allylic alcohols.

Science.gov (United States)

Adam, W; Humpf, H U; Roschmann, K J; Saha-Möller, C R

2001-08-24

A set of aryl-substituted allylic alcohols rac-2 has been epoxidized by chiral Mn(salen*) complexes 1 as the catalyst and iodosyl benzene (PhIO) as the oxygen source. Whereas one enantiomer of the allylic alcohol 2 is preferentially epoxidized to give the threo- or cis-epoxy alcohol 3 (up to 80% ee) as the main product (dr up to >95:5), the other enantiomer of 2 is enriched (up to 53% ee). In the case of 1,1-dimethyl-1,2-dihydronaphthalen-2-ol (2c), the CH oxidation to the enone 4c proceeds enantioselectively and competes with the epoxidation. The absolute configurations of the allylic alcohols 2 and their epoxides 3 have been determined by chemical correlation or CD spectroscopy. The observed diastereo- and enantioselectivities in the epoxidation reactions are rationalized in terms of a beneficial interplay between the hydroxy-directing effect and the attack along the Katsuki trajectory.

Free radical reaction pathway, thermohemistry of peracetic acid homolysis and its application for phenol degradation: spectroscopic sti=udy and quantum chemistry calculations

NARCIS (Netherlands)

Rokhina, E.V.; Makarova, K.; Golovina, E.A.; As, van H.; Virkutyte, J.

2010-01-01

The homolysis of peracetic acid (PAA) as a relevant source of free radicals (e.g., •OH) was studied in detail. Radicals formed as a result of chain radical reactions were detected with electron spin resonance and nuclear magnetic resonance spin trapping techniques and subsequently identified by

Enantioselective Cytotoxicity Profile of o,p’-DDT in PC 12 Cells

Science.gov (United States)

Zhang, Chunlong; Wen, Yuezhong; Liu, Weiping

2012-01-01

Background The continued uses of dichlordiphenyltrichloroethane (DDT) for indoor vector control in some developing countries have recently fueled intensive debates toward the global ban of this persistent legacy contaminant. Current approaches for ecological and health risk assessment has ignored the chiral nature of DDT. In this study by employing an array of cytotoxicity related endpoints, we investigated the enantioselective cytotoxicity of o,p’-DDT. Principal Findings we demonstrated for the first time that R-(−)-o,p’-DDT caused more neuron cell death by inducing more severe oxidative stress, which selectively imbalanced the transcription of stress-related genes (SOD1, SOD2, HSP70) and enzyme (superoxide dismutase and lactate dehydrogenase) activities, and greater cellular apoptosis compared to its enantiomer S-(+)-o,p’-DDT at the level comparable to malaria area exposure (parts per million). We further elucidated enantioselective modes of action using microarray combined with enzyme-linked immunosorbent assay. The enantioselective apoptosis might involve three signaling pathways via caspase 3, tumor protein 53 (p53) and NFkB. Conclusions Based on DDT stereochemistry and results reported for other chiral pesticides, our results pointed to the same directional enantioselectivity of chiral DDT toward mammalian cells. We proposed that risk assessment on DDT should consider the enantiomer ratio and enantioselectivities. PMID:22937105

Enantioselective carbenoid insertion into C(sp3–H bonds

Directory of Open Access Journals (Sweden)

J. V. Santiago

2016-05-01

Full Text Available The enantioselective carbenoid insertion into C(sp3–H bonds is an important tool for the synthesis of complex molecules due to the high control of enantioselectivity in the formation of stereogenic centers. This paper presents a brief review of the early issues, related mechanistic studies and recent applications on this chemistry area.

Direct, enantioselective α-alkylation of aldehydes using simple olefins.

Science.gov (United States)

Capacci, Andrew G; Malinowski, Justin T; McAlpine, Neil J; Kuhne, Jerome; MacMillan, David W C

2017-11-01

Although the α-alkylation of ketones has already been established, the analogous reaction using aldehyde substrates has proven surprisingly elusive. Despite the structural similarities between the two classes of compounds, the sensitivity and unique reactivity of the aldehyde functionality has typically required activated substrates or specialized additives. Here, we show that the synergistic merger of three catalytic processes-photoredox, enamine and hydrogen-atom transfer (HAT) catalysis-enables an enantioselective α-aldehyde alkylation reaction that employs simple olefins as coupling partners. Chiral imidazolidinones or prolinols, in combination with a thiophenol, iridium photoredox catalyst and visible light, have been successfully used in a triple catalytic process that is temporally sequenced to deliver a new hydrogen and electron-borrowing mechanism. This multicatalytic process enables both intra- and intermolecular aldehyde α-methylene coupling with olefins to construct both cyclic and acyclic products, respectively. With respect to atom and step-economy ideals, this stereoselective process allows the production of high-value molecules from feedstock chemicals in one step while consuming only photons.

Enantioselective Copper-Catalyzed Oxy-Alkynylation of Diazo Compounds.

Science.gov (United States)

Hari, Durga Prasad; Waser, Jerome

2017-06-28

Enantioselective catalytic methods allowing the addition of both a nucleophile and an electrophile onto diazo compounds give a fast access into important building blocks. Herein, we report the highly enantioselective oxyalkynylation of diazo compounds using ethynylbenziodoxol-(on)e reagents and a simple copper bisoxazoline catalyst. The obtained α-benzoyloxy propargylic esters are useful building blocks, which are difficult to synthesize in enantiopure form using other methods. The obtained products could be efficiently transformed into vicinal diols and α-hydroxy propargylic esters without loss in enantiopurity.

A combined continuous microflow photochemistry and asymmetric organocatalysis approach for the enantioselective synthesis of tetrahydroquinolines

Directory of Open Access Journals (Sweden)

Erli Sugiono

2013-11-01

Full Text Available A continuous-flow asymmetric organocatalytic photocyclization–transfer hydrogenation cascade reaction has been developed. The new protocol allows the synthesis of tetrahydroquinolines from readily available 2-aminochalcones using a combination of photochemistry and asymmetric Brønsted acid catalysis. The photocylization and subsequent reduction was performed with catalytic amount of chiral BINOL derived phosphoric acid diester and Hantzsch dihydropyridine as hydrogen source providing the desired products in good yields and with excellent enantioselectivities.

Enantioselective changes in oxidative stress and toxin release in Microcystis aeruginosa exposed to chiral herbicide diclofop acid

Energy Technology Data Exchange (ETDEWEB)

Ye, Jing [School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418 (China); MOE Key Lab of Environmental Remediation and Ecosystem Health, College of Natural Research and Environmental Sciences, Zhejiang University, Hangzhou 310058 (China); Zhang, Ying [Department of Environmental Science, East China Normal University, Shanghai 200241 (China); Chen, Shengwen [School of Urban Development and Environment Engineering, Shanghai Second Polytechnic University, Shanghai 201209 (China); Liu, Chaonan; Zhu, Yongqiang [School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418 (China); Liu, Weiping, E-mail: wliu@zju.edu.cn [MOE Key Lab of Environmental Remediation and Ecosystem Health, College of Natural Research and Environmental Sciences, Zhejiang University, Hangzhou 310058 (China)

2014-01-15

Highlights: •The first study on enantioselective oxidative stress and toxin release from Microcystis aeruginosa. •Provide information for the R-enantiomer poses more oxidative stress than the S-enantiomer. •Lifecycle analysis of chiral pollutants needs more attention in environmental assessment. -- Abstract: Enantioselective oxidative stress and toxin release from Microcystis aeruginosa after exposure to the chiral herbicide diclofop acid were investigated. Racemic diclofop acid, R-diclofop acid and S-diclofop acid induced reactive oxygen species (ROS) generation, increased the concentration of malondialdehyde (MDA), enhanced the activity of superoxide dismutase (SOD) and triggered toxin release in M. aeruginosa to varying degrees. The increase in MDA concentration and SOD activity in M. aeruginosa occurred sooner after exposure to diclofop acid than when the cyanobacteria was exposed to either the R- and the S-enantiomer. In addition, enantioselective toxicity of the enantiomers was observed. The R-enantiomer trigged more ROS generation, more SOD activity and more toxin synthesis and release in M. aeruginosa cells than the S-enantiomer. Diclofop acid and its R-enantiomer may collapse the transmembrane proton gradient and destroy the cell membrane through lipid peroxidation and free radical oxidation, whereas the S-enantiomer did not demonstrate such action. R-diclofop acid inhibits the growth of M. aeruginosa in the early stage, but ultimately induced greater toxin release, which has a deleterious effect on the water column. These results indicate that more comprehensive study is needed to determine the environmental safety of the enantiomers, and application of chiral pesticides requires more direct supervision and training. Additionally, lifecycle analysis of chiral pollutants in aquatic system needs more attention to aide in the environmental assessment of chiral pesticides.

Comprehensive Theoretical Studies on the Reaction of 1-Bromo-3,3,3-trifluoropropene with OH Free Radicals

Directory of Open Access Journals (Sweden)

Yan Tian

2013-07-01

Full Text Available The potential energy surfaces (PES for the reaction of 1-bromo-3,3,3-trifluoropropene (CF3CHCBrH with hydroxyl (OH free radicals is probed theoretically at the CCSD/aug-cc-pVDZ//B3LYP/6-311++G(d,p level of theory. All the possible stationary and first-order saddle points along the reaction paths were verified by the vibrational analysis. The calculations account for all the product channels. Based on the calculated CCSD/aug-cc-pVDZ potential energy surface, the possible reaction mechanism is discussed. Six distinct reaction pathways of 1-bromo-3,3,3-trifluoropropene (BTP with OH are investigated. The geometries, reaction enthalpies and energy barriers are determined. Canonical transition-state theory with Wigner tunneling correction was used to predict the rate constants for the temperature range of 290–3,000 K without any artificial adjustment, and the computed rate constants for elementary channels can be accurately fitted with three-parameter Arrhenius expressions. OH addition reaction channel and the H atom abstraction channels related to the carbon-carbon double bond are found to be the main reaction channels for the reaction of 1-bromo-3,3,3-trifluoropropene (CF3CHCBrH with hydroxyl (OH free radicals while the products leading to CF3CHCH + BrOH and COHF2CHCBrH + F play a negligible role.

Isoindolinones as Michael Donors under Phase Transfer Catalysis: Enantioselective Synthesis of Phthalimidines Containing a Tetrasubstituted Carbon Stereocenter

Directory of Open Access Journals (Sweden)

Francesco Scorzelli

2015-05-01

Full Text Available Readily available chiral ammonium salts derived from cinchona alkaloids have proven to be effective phase transfer catalysts in the asymmetric Michael reaction of 3-substituted isoindolinones. This protocol provides a convenient method for the construction of valuable asymmetric 3,3-disubstituted isoindolinones in high yields and  moderate to good enantioselectivity. Diastereoselectivity was also investigated in the construction of contiguous tertiary and quaternary stereocenters. The use of acrolein as Michael acceptor led to an interesting tricyclic derivative, a pyrroloisoindolinone analogue, via a tandem conjugated addition/cyclization reaction.

Kinetics of the reaction of CH3O2 radicals with NO2

DEFF Research Database (Denmark)

Wallington, T.J.; Nielsen, O.J.; Sehested, K.

1999-01-01

The kinetics of the gas-phase reaction of CH3O2 radicals with NO2 were studied at 295 K in 0.5-14 arm of SF6 diluent using pulse radiolysis combined with time-resolved UV-VIS spectroscopy. Rate data were obtained by following the loss of CH3O2 using a monitoring wavelength of 260 nm. The results...

Atmospheric chemistry of perfluorinated carboxylic acids: Reaction with OH radicals and atmospheric lifetimes

DEFF Research Database (Denmark)

Hurley, MD; Andersen, Mads Peter Sulbæk; Wallington, TJ

2004-01-01

Relative rate techniques were used to study the kinetics of the reactions of OH radicals with a homologous series of perfluorinated acids, F(CF2)(n)COOH (n = 1, 2, 3, 4), in 700 Torr of air at 296 +/- 2 K. For n > 1, the length of the F(CF2)(n) group had no discernible impact on the reactivity of...

Free radicals in health and disease

International Nuclear Information System (INIS)

Gonet, B.

1993-01-01

Free radicals appear in the cells as the result of exogenic factors (ionizing radiation, UV) or reactions naturally occurring in the cell. Free radical reactions may cause destruction of macromolecules (DNA, lipids, proteins). Free radical pathology is important in many diseases and aging processes in organisms

CO2 as a C1-organic building block: Enantioselective electrocarboxylation of aromatic ketones with CO2catalyzed by cinchona alkaloids under mild conditions

International Nuclear Information System (INIS)

Chen, Bao-Li; Tu, Zhuo-Ying; Zhu, Hong-Wei; Sun, Wen-Wen; Wang, Huan; Lu, Jia-Xing

2014-01-01

Highlights: •Cinchona alkaloids catalysis achieve enantioselective electrocarboxylation of racemic aromatic ketones. •The applications of CO 2 enantioselective electrochemical fixation into optically active hydroxyl carboxylic acids have been expanded. •The applications of alkaloids have been expanded. •The applications of asymmetric synthesis by electrochemical methodology have been expanded. -- Abstract: The enantioselective electrocarboxylation of pro-chiral aromatic ketones (2-acetonaphthone, 1-(6-methoxy-2-naphthyl)ethanone, 1-(4-methoxy-1-naphthyl)ethanone) with atmospheric pressure of CO 2 catalyzed by cinchona alkaloids in the presence of phenol was investigated in an undivided cell for the first time to give optically active 2-hydroxy-2-arylpropionic acid. For the model compound 2-acetonaphthone, the influence of various reaction conditions, such as cathode material, current density, catalyst type, ratio of proton to catalyst and catalyst quantity, on the enantiomeric excesses (ee) and yield has been investigated. Under the optimized conditions of 2-acetonaphthone, all the aromatic ketones examined are converted into corresponding optically active 2-hydroxy-2-arylpropionic acids in moderate yield (32.2% - 41.3%) and ee (48.1% - 48.6%). In addition, the electrochemical behavior of 2-acetonaphthone has been studied by cyclic voltammetry (CV) in the absence and presence of CO 2 . Moreover, the probable reaction pathway was proposed accordingly

The reactions of SO3 with HO2 radical and H2O...HO2 radical complex. Theoretical study on the atmospheric formation of HSO5 and H2SO4.

Science.gov (United States)

Gonzalez, Javier; Torrent-Sucarrat, Miquel; Anglada, Josep M

2010-03-07

The influence of a single water molecule on the gas-phase reactivity of the HO(2) radical has been investigated by studying the reactions of SO(3) with the HO(2) radical and with the H(2)O...HO(2) radical complex. The naked reaction leads to the formation of the HSO(5) radical, with a computed binding energy of 13.81 kcal mol(-1). The reaction with the H(2)O...HO(2) radical complex can give two different products, namely (a) HSO(5) + H(2)O, which has a binding energy that is computed to be 4.76 kcal mol(-1) more stable than the SO(3) + H(2)O...HO(2) reactants (Delta(E + ZPE) at 0K) and an estimated branching ratio of about 34% at 298K and (b) sulfuric acid and the hydroperoxyl radical, which is computed to be 10.51 kcal mol(-1) below the energy of the reactants (Delta(E + ZPE) at 0K), with an estimated branching ratio of about 66% at 298K. The fact that one of the products is H(2)SO(4) may have relevance in the chemistry of the atmosphere. Interestingly, the water molecule acts as a catalyst, [as it occurs in (a)] or as a reactant [as it occurs in (b)]. For a sake of completeness we have also calculated the anharmonic vibrational frequencies for HO(2), HSO(5), the HSO(5)...H(2)O hydrogen bonded complex, H(2)SO(4), and two H(2)SO(4)...H(2)O complexes, in order to help with the possible experimental identification of some of these species.

One-pot cascade michael.cyclization reactions of ο-hydroxycinnamaldehydes: Synthesis of functionalized 2,3-dihydrobenzofuranes

Energy Technology Data Exchange (ETDEWEB)

Gwon, Sung Hyuk; Kim, Sunggon [Kyonggi Univ., Suwon (Korea, Republic of)

2012-04-15

We described the cascade Michael.cyclization reaction of o-hydroxycinnamaldehydes with diethyl α-bromomalonate promoted by potassium carbonate. The reactions provided functionalized 2,3-dihydrobenzofurans in good yields for a variety of o-hydroxyaromatic α,β-unsaturated aldehydes. Current work focuses on expanding the scope of this reaction to other substrates such as sulfur yields, and on developing an efficient catalytic asymmetric variant. 2,3-Dihydrobenzofurans are found in numerous biologically active natural products and synthetic compounds. These are an attractive type of oxygenated compound because their basic core skeleton is present in neolignans, pterocarpans, and synthetic drugs used in the treatment of pulmonary hypertension, atherosclerotic peripheral arterial disease, and central nervous system trauma and ischemia. Owing to the importance of their structures, numerous synthetic methods for 2,3-dihydrobenzofurans have been developed, primarily: radical cyclizations, Lewis acid promoted reactions, anionic cyclizations, and transition-metal catalyzed processes. However, these methods cannot be generalized as much as is desirable, and they yielded poor chemo- and/or stereoselectivities. Hence, the development of an efficient enantioselective synthetic method for obtaining 2,3-dihydrobenzofuran scaffolds attracted our attention.

One-pot cascade michael.cyclization reactions of ο-hydroxycinnamaldehydes: Synthesis of functionalized 2,3-dihydrobenzofuranes

International Nuclear Information System (INIS)

Gwon, Sung Hyuk; Kim, Sunggon

2012-01-01

We described the cascade Michael.cyclization reaction of o-hydroxycinnamaldehydes with diethyl α-bromomalonate promoted by potassium carbonate. The reactions provided functionalized 2,3-dihydrobenzofurans in good yields for a variety of o-hydroxyaromatic α,β-unsaturated aldehydes. Current work focuses on expanding the scope of this reaction to other substrates such as sulfur yields, and on developing an efficient catalytic asymmetric variant. 2,3-Dihydrobenzofurans are found in numerous biologically active natural products and synthetic compounds. These are an attractive type of oxygenated compound because their basic core skeleton is present in neolignans, pterocarpans, and synthetic drugs used in the treatment of pulmonary hypertension, atherosclerotic peripheral arterial disease, and central nervous system trauma and ischemia. Owing to the importance of their structures, numerous synthetic methods for 2,3-dihydrobenzofurans have been developed, primarily: radical cyclizations, Lewis acid promoted reactions, anionic cyclizations, and transition-metal catalyzed processes. However, these methods cannot be generalized as much as is desirable, and they yielded poor chemo- and/or stereoselectivities. Hence, the development of an efficient enantioselective synthetic method for obtaining 2,3-dihydrobenzofuran scaffolds attracted our attention

Alkyl hydrogen atom abstraction reactions of the CN radical with ethanol

Science.gov (United States)

Athokpam, Bijyalaxmi; Ramesh, Sai G.

2018-04-01

We present a study of the abstraction of alkyl hydrogen atoms from the β and α positions of ethanol by the CN radical in solution using the Empirical Valence Bond (EVB) method. We have built separate 2 × 2 EVB models for the Hβ and Hα reactions, where the atom transfer is parameterized using ab initio calculations. The intra- and intermolecular potentials of the reactant and product molecules were modelled with the General AMBER Force Field, with some modifications. We have carried out the dynamics in water and chloroform, which are solvents of contrasting polarity. We have computed the potential of mean force for both abstractions in each of the solvents. They are found to have a small and early barrier along the reaction coordinate with a large energy release. Analyzing the solvent structure around the reaction system, we have found two solvents to have little effect on either reaction. Simulating the dynamics from the transition state, we also study the fate of the energies in the HCN vibrational modes. The HCN molecule is born vibrationally hot in the CH stretch in both reactions and additionally in the HCN bends for the Hα abstraction reaction. In the early stage of the dynamics, we find that the CN stretch mode gains energy at the expense of the energy in CH stretch mode.

Generation of counter ion radical (Br2(•-)) and its reactions in water-in-oil (CTAB or CPB)/n-butanol/cyclohexane/water) microemulsion.

Science.gov (United States)

Guleria, Apurav; Singh, Ajay K; Sarkar, Sisir K; Mukherjee, Tulsi; Adhikari, Soumyakanti

2011-09-15

Herein we report the generation of counterion radicals and their reactions in quaternary water-in-oil microemulsion. Hydrated electrons in the microemulsion CTAB/H(2)O/n-butanol/cyclohexane have a remarkably short half-life (∼1 μs) and lower yield as compared to that in the pure water system. Electrons are solvated in two regions: one is the water core and other the interface; however, the electrons in the water core have a shorter half-life than those in the interface. The decay of the solvated electrons in the interface is found to be water content dependent and it has been interpreted in terms of increased interfacial fluidity with the increase in water content of the microemulsion. Interestingly another species, dibromide radical anion (Br(2)(•-)) in CTAB and CPB microemulsions have been observed after the electron beam irradiation. Assuming that the extinction coefficient of the radicals is the same as that in the aqueous solution, the yields of the radicals per 100 eV are 0.29 and 0.48 for the Br(2)(•-) radical in CTAB and CPB containing microemulsions (W(0) = 40), respectively, under N(2)O saturated conditions. Further, we intended to study electron transfer reactions, which occur at and through the interface. The reaction of the Br(2)(•-) radical anion with ABTS [2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)] has been studied to generate the ABTS radical in the water core, and further, its reaction has been investigated with the water-insoluble molecule vitamin E (tocopherol) and water-soluble vitamin C (ascorbic acid). In the present study, we were able to show that, even for molecules which are completely insoluble in water, ABTS scavenging assay is possible by pulse radiolysis technique. Furthermore, these results show that it is possible to follow the reaction of the hydrated inorganic radical with solutes dissolved in the organic phase in a microemulsion without use of a phase transfer catalyst. © 2011 American Chemical Society

Kinetics of nitrosamine and amine reactions with NO3 radical and ozone related to aqueous particle and cloud droplet chemistry

Science.gov (United States)

Weller, Christian; Herrmann, Hartmut

2015-01-01

Aqueous phase reactivity experiments with the amines dimethylamine (DMA), diethanolamine (DEA) and pyrrolidine (PYL) and their corresponding nitrosamines nitrosodimethylamine (NDMA), nitrosodiethanolamine (NDEA) and nitrosopyrrolidine (NPYL) have been performed. NO3 radical reaction rate coefficients for DMA, DEA and PYL were measured for the first time and are 3.7 × 105, 8.2 × 105 and 8.7 × 105 M-1 s-1, respectively. Rate coefficients for NO3 + NDMA, NDEA and NPYL are 1.2 × 108, 2.3 × 108 and 2.4 × 108 M-1 s-1. Compared to OH radical rate coefficients for reactions with amines, the NO3 radical will most likely not be an important oxidant but it is a potential nighttime oxidant for nitrosamines in cloud droplets or deliquescent particles. Ozone is unreactive towards amines and nitrosamines and upper limits of rate coefficients suggest that aqueous ozone reactions are not important in atmospheric waters.

Rate constants for the reaction of OH radicals with 1-chloroalkanes at 295 K

DEFF Research Database (Denmark)

Markert, F.; Nielsen, O.J.

1992-01-01

The rate constants for the reaction of OH radicals with a series of 1-chloroalkanes were measured at 295 K and at a total pressure of 1 atm. The rate constants were obtained by using the absolute technique of pulse radiolysis combined with kinetic UV-spectroscopy. The results are discussed in terms...

Recyclable enantioselective catalysts based on copper(II) complexes of 2-(pyridine-2-yl)imidazolidine-4-thione: their application in asymmetric Henry reactions

Czech Academy of Sciences Publication Activity Database

Nováková, G.; Drabina, P.; Frumarová, Božena; Sedlák, M.

2016-01-01

Roč. 358, č. 15 (2016), s. 2541-2552 ISSN 1615-4150 Institutional support: RVO:61389013 Keywords : asymmetric catalysis * enantioselectivity * heterogeneous catalysis Subject RIV: CC - Organic Chemistry Impact factor: 5.646, year: 2016

Studies of radiation-produced radicals and radical ions. Progress report, June 1, 1981-August 31, 1982

International Nuclear Information System (INIS)

Williams, T.F.

1982-01-01

The discovery and characterization of novel radical ions produced by the γ irradiation of solids continues to be a fertile field for investigation. This Progress Report describes the generation and ESR identification of several new paramagnetic species, some of which have long been sought as important intermediates in radiation chemistry. We have also contributed to a general theoretical problem in ESR spectroscopy. Solid-state studies of electron attachment reactions, both non-dissociative and dissociative, reveal interesting structural and chemical information about the molecular nature of these processes for simple compounds. In particular, ESR measurements of the spin distribution in the products allow a fairly sharp distinction to be drawn between radical anions and radical-anion pairs or adducts. Dimer radical anion formation can also take place but the crystal structure plays a role in this process, as expected. Some radical anions undergo photolysis to give radical-anion pairs which may then revert back to the original radical anion by a thermal reaction. The chemistry of these reversible processes is made more intricate by a competing reaction in which the radical abstracts a hydrogen atom from a neighboring molecule. However, the unraveling of this complication has also served to extend our knowledge of the role of quantum tunneling in chemical reactions. The results of this investigation testify to the potential of solid-state techniques for the study of novel and frangible radical ions. Progress in this field shows no sign of abating, as witness the recent discovery of perfluorocycloalkane radical anions and alkane radical cations

Evaluation of achiral templates with fluxional Brønsted basic substituents in enantioselective conjugate additions.

Science.gov (United States)

Adachi, Shinya; Takeda, Norihiko; Sibi, Mukund P

2014-12-19

Enantioselective conjugate addition of malononitrile to pyrazolidinone-derived enoates proceeds in excellent yields and high enantioselectivities. A comparison of fluxional substituents with and without a Brønsted basic site and their impact on selectivity is detailed. Molecular sieves as an additive were found to be essential to achieve high enantioselectivity.

Palladium-Catalyzed Asymmetric Conjugate Addition of Arylboronic Acids to Five-, Six-, and Seven-Membered β-Substituted Cyclic Enones: Enantioselective Construction of All-Carbon Quaternary Stereocenters

KAUST Repository

Kikushima, Kotaro; Holder, Jeffrey C.; Gatti, Michele; Stoltz, Brian M.

2011-01-01

The first enantioselective Pd-catalyzed construction of all-carbon quaternary stereocenters via 1,4-addition of arylboronic acids to β-substituted cyclic enones is reported. Reaction of a wide range of arylboronic acids and cyclic enones using a

Reaction of gadolinium chelates with ozone and hydroxyl radicals.

Science.gov (United States)

Cyris, Maike; Knolle, Wolfgang; Richard, Jessica; Dopp, Elke; von Sonntag, Clemens; Schmidt, Torsten C

2013-09-03

Gadolinium chelates are used in increasing amounts as contrast agents in magnetic resonance imaging, and their fate in wastewater treatment has recently become the focus of research. Oxidative processes, in particular the application of ozone, are currently discussed or even implemented for advanced wastewater treatment. However, reactions of the gadolinium chelates with ozone are not yet characterized. In this study, therefore, rate constants with ozone were determined for the three commonly used chelates Gd-DTPA, Gd-DTPA-BMA, and Gd-BT-DO3A, which were found to be 4.8 ± 0.88, 46 ± 2.5, and 24 ± 1.5 M(-1) s(-1), respectively. These low rate constants indicate that a direct reaction with ozone in wastewater is negligible. However, application of ozone in wastewater leads to substantial yields of (•)OH. Different methods have been applied and compared for determination of k((•)OH+Gd chelate). From rate constants determined by pulse radiolysis experiments (k((•)OH+Gd-DTPA) = 2.6 ± 0.2 × 10(9) M(-1) s(-1), k((•)OH+Gd-DTPA-BMA) = 1.9 ± 0.7 × 10(9) M(-1) s(-1), k((•)OH+Gd-BT-DO3A) = 4.3 ± 0.2 × 10(9) M(-1) s(-1)), it is concluded that a reaction in wastewater via (•)OH radicals is feasible. Toxicity has been tested for educt and product mixtures of both reactions. Cytotoxicity (MTT test) and genotoxicity (micronuclei assay) were not detectable.

Flexible Enantioselectivity of Tryptophanase Attributable to Benzene Ring in Heterocyclic Moiety of D-Tryptophan

Directory of Open Access Journals (Sweden)

Akihiko Shimada

2012-05-01

Full Text Available The invariance principle of enzyme enantioselectivity must be absolute because it is absolutely essential to the homochiral biological world. Most enzymes are strictly enantioselective, and tryptophanase is one of the enzymes with extreme absolute enantioselectivity for L-tryptophan. Contrary to conventional knowledge about the principle, tryptophanase becomes flexible to catalyze D-tryptophan in the presence of diammonium hydrogenphosphate. Since D-amino acids are ordinarily inert or function as inhibitors even though they are bound to the active site, the inhibition behavior of D-tryptophan and several inhibitors involved in this process was examined in terms of kinetics to explain the reason for this flexible enantioselectivity in the presence of diammonium hydrogenphosphate. Diammonium hydrogenphosphate gave tryptophanase a small conformational change so that D-tryptophan could work as a substrate. As opposed to other D-amino acids, D-tryptophan is a very bulky amino acid with a benzene ring in its heterocyclic moiety, and so we suggest that this structural feature makes the catalysis of D-tryptophan degradation possible, consequently leading to the flexible enantioselectivity. The present results not only help to understand the mechanism of enzyme enantioselectivity, but also shed light on the origin of homochirality.

P(O)R2-Directed Enantioselective C-H Olefination toward Chiral Atropoisomeric Phosphine-Olefin Compounds.

Science.gov (United States)

Li, Shi-Xia; Ma, Yan-Na; Yang, Shang-Dong

2017-04-07

An effective synthesis of chiral atropoisomeric biaryl phosphine-olefin compounds via palladium-catalyzed enantioselective C-H olefination has been developed for the first time. The reactions are operationally simple, tolerate wide functional groups, and have a good ee value. Notably, P(O)R 2 not only acts as the directing group to direct C-H activation in order to make a useful ligand but also serves to facilitate composition of the product in a useful manner in this transformation.

Effect of reagents and medium nature on direction of cation-radical transformations in the reaction of diarylamines with nitrosonium borofluoride

International Nuclear Information System (INIS)

Koshechko, V.G.; Inozemtsev, A.N.; Pokhodenko, V.O.

1983-01-01

Diphenylamine and 4, 4'-dimethoxydiphenylamine in acetonitrile are oxidized monoelectronically by NOBF 4 . On the 4, 4'-dimethoxydiphenylamine example a maximum current concentration of intermediate cation-radicals formed in this reaction is measured. Cation-radicals in acetonitrile get dimeric transforming into products of oxidizing condensation of amines, with no respective nitrozamines being observed. Nitrosamines production on through cation-radical-NO recombination is realized but in the presence of protonoacceptor solvents, in particular pyridine

Sequential rhodium/palladium catalysis: enantioselective formation of dihydroquinolinones in the presence of achiral and chiral ligands.

Science.gov (United States)

Zhang, Lei; Qureshi, Zafar; Sonaglia, Lorenzo; Lautens, Mark

2014-12-08

Compatible combinations of achiral and chiral ligands can be used in rhodium/palladium catalysis to achieve highly enantioselective domino reactions. The difference in rates of catalysis and minimal effects of ligand interference confer control in the domino sequence. The "all-in-one" 1,4-conjugate arylation and C-N cross-coupling through sequential Rh/Pd catalysis provides access to enantioenriched dihydroquinolinone building blocks. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Enantioselective Organocatalysis in Microreactors: Continuous Flow Synthesis of a (S-Pregabalin Precursor and (S-Warfarin

Directory of Open Access Journals (Sweden)

Riccardo Porta

2015-08-01

Full Text Available Continuous flow processes have recently emerged as a powerful technology for performing chemical transformations since they ensure some advantages over traditional batch procedures. In this work, the use of commercially available and affordable PEEK (Polyetheretherketone and PTFE (Polytetrafluoroethylene HPLC (High Performance Liquid Chromatography tubing as microreactors was exploited to perform organic reactions under continuous flow conditions, as an alternative to the commercial traditional glass microreactors. The wide availability of tubing with different sizes allowed quickly running small-scale preliminary screenings, in order to optimize the reaction parameters, and then to realize under the best experimental conditions a reaction scale up for preparative purposes. The gram production of some Active Pharmaceutical Ingredients (APIs such as (S-Pregabalin and (S-Warfarin was accomplished in short reaction time with high enantioselectivity, in an experimentally very simple procedure.

Radical-Mediated Enzymatic Polymerizations

Science.gov (United States)

Zavada, Scott R.; Battsengel, Tsatsral; Scott, Timothy F.

2016-01-01

Polymerization reactions are commonly effected by exposing monomer formulations to some initiation stimulus such as elevated temperature, light, or a chemical reactant. Increasingly, these polymerization reactions are mediated by enzymes―catalytic proteins―owing to their reaction efficiency under mild conditions as well as their environmental friendliness. The utilization of enzymes, particularly oxidases and peroxidases, for generating radicals via reduction-oxidation mechanisms is especially common for initiating radical-mediated polymerization reactions, including vinyl chain-growth polymerization, atom transfer radical polymerization, thiol–ene step-growth polymerization, and polymerization via oxidative coupling. While enzyme-mediated polymerization is useful for the production of materials intended for subsequent use, it is especially well-suited for in situ polymerizations, where the polymer is formed in the place where it will be utilized. Such polymerizations are especially useful for biomedical adhesives and for sensing applications. PMID:26848652

Reaction of hypochlorite with amino acids and peptides : EPR evidence for rapid rearrangement and fragmentation of nitrogen-centred radicals

International Nuclear Information System (INIS)

Hawkins, C.L.; Davies, M.J.

1998-01-01

Various amino acid side chains have been shown to be particularly susceptible to attack and modification by hypochlorite (HOCl). It is known that tyrosine is readily chlorinated by HOCl to give 3-chlorotyrosine and this product has been employed as a marker of HOCl-mediated damage to proteins. Cysteine and methionine react rapidly with HOCl to give oxy acids and cystine (from cysteine) and sulphoxides (from methionine). Lysine and amino acids which lack the above functional groups also react with HOCl via the free amino group which results in the generation of unstable chloramine intermediates; subsequent decomposition of these species gives NH 3 , CO 2 and aldehydes. While the products of reaction of HOCl with amino acids and peptides are reasonably well characterised, the mechanism(s) by which these products arise is less well understood. Electron paramagnetic resonance (EPR) spectroscopy with spin trapping and UV/visible spectroscopy has been employed to examine the reaction of HOCl with amino acids and some small peptides. Reaction of HOCl with N-acetyl amino acids or small peptides gives radicals predominantly at α-carbon sites via reaction at N-terminal free amino groups or amide (peptide) bonds. It is proposed that these carbon-centred radicals are produced as a result of the rearrangement of initial nitrogen-centred radicals formed on cleavage of the N-CI bond of the chloramine/chloramide species by a 1,2-shift reaction

Reaction paths and rate constants of the reaction of hydroxyl radicals with environmental species under tropospheric conditions

International Nuclear Information System (INIS)

Leonard, C.; Wahner, A.; Zetzsch, C.

1987-01-01

The uv-laser absorption technique in a multipath cell (with excimer-laser photolysis for radical production) is used to investigate the rate constants of the reaction of OH with carbon monoxide. The pressure dependence and the influence of collision partners (measurements in pure oxygen up to one atmosphere) of this important atmospheric chemical reaction are determined. In the kinetic measurements detection limits of 10 7 OH cm -3 are reached with millisecond time resolution. Furthermore the application of the cw-Laser for stationary OH measurements (for example in smog chambers or the free troposphere) is described. The possibilities and limits of different detection methods are discussed with respect to of noise spectra. Modifications of the apparatus with a frequency modulation technique are presented, with an extrapolated detection limit of 10 5 OH cm -3 . (orig.) With 43 refs., 16 figs [de

Kinetic of the gas-phase reactions of OH radicals and Cl atoms with Diethyl Ethylphosphonate and Triethyl Phosphate

KAUST Repository

Laversin, H.

2015-11-30

In this paper, the relative-rate technique has been used to obtain rate coefficients for the reaction of two organophosphorus compounds: Triethyl phosphate (TEP) and Diethyl ethylphosphonate (DEEP) with OH radicals and Cl atoms at atmospheric pressure and at different temperatures. The calculated rate constants were fitted to the Arrhenius expression over the temperature range 298 – 352 K. The following expressions (in cm3molecule-1s-1) were obtained for the reactions of OH and CL with DEEP and TEP: kOH+DEEP= (7.84±0.65)x10-14exp((1866±824)/T), kOH+TEP = (6.54±0.42)x10-14exp((1897±626)/T), kCl+DEEP = (5.27± 0.80)x10−11exp(765±140/T) and kCl+TEP = (5.23± 0.80)x10−11exp(736± 110/T). These results show that the reaction of the studied compounds with Cl atoms proceeds more rapidly than that with OH radicals. The related tropospheric lifetimes suggest that once emitted into the atmosphere, TEP and DEEP can be removed within a few hours in areas close to their emission sources. TEP and DEEP are principally removed by OH radicals. However, in coastal areas where the Cl atoms’ concentration is higher, TEP and DEEP removal by reaction with Cl atoms could be a competitive process.

Comments on the reactions of carbohydrate peroxy radicals in relation to the lyoluminescent behaviour of irradiated carbohydrates

International Nuclear Information System (INIS)

Baugh, P.J.; Mahjani, M.G.

1977-01-01

Reference is made to recent work on lyoluminescence: the emission of visible light from irradiated tissue equivalent solids such as carbohydrates when dissolved in aqueous solutions (Atari et al., Radiat. Effects; 17:45(1973); and ibid.; 20: 135 (1973); and Baugh et al., Int.J.Radiat.Phys. Chem.(in press)). In the present communication the consequences of the fast elimination of the hydroperoxy radicals from carbohydrate peroxy radicals are considered in a further study of the chemical reactions involved. (U.K.)

Enantioselective solvent-free Robinson annulation reactions

Indian Academy of Sciences (India)

Unknown

solvents to effect an asymmetric synthesis is an important step forward towards ... In continuation of our preliminary communication 2, we wish to ..... formation of chiral enamine 74 from the reaction of S-proline with pro-R carbonyl group.

Heterogeneous kinetics, products, and mechanisms of ferulic acid particles in the reaction with NO3 radicals

Science.gov (United States)

Liu, Changgeng; Zhang, Peng; Wen, Xiaoying; Wu, Bin

2017-03-01

Methoxyphenols, as an important component of wood burning, are produced by lignin pyrolysis and considered to be the potential tracers for wood smoke emissions. In this work, the heterogeneous reaction between ferulic acid particles and NO3 radicals was investigated. Six products including oxalic acid, 4-vinylguaiacol, vanillin, 5-nitrovanillin, 5-nitroferulic acid, and caffeic acid were confirmed by gas chromatography-mass spectrometry (GC-MS). In addition, the reaction mechanisms were proposed and the main pathways were NO3 electrophilic addition to olefin and the meta-position to the hydroxyl group. The uptake coefficient of NO3 radicals on ferulic acid particles was 0.17 ± 0.02 and the effective rate constant under experimental conditions was (1.71 ± 0.08) × 10-12 cm3 molecule-1 s-1. The results indicate that ferulic acid degradation by NO3 can be an important sink at night.

Kinetics of the reaction of hydroxyl radicals with ethane and a series of Cl- and F-substituted methanes at 300-400degK

International Nuclear Information System (INIS)

Bjarnov, E.; Munk, J.; Nielsen, O.J.; Pagsberg, P.; Sillesen, A.

1982-04-01

Gas phase reactions of hydroxyl radicals with ethane and a series of Cl- and F-substituted methanes were studied at atmospheric pressure and over the temperature range 300-400degK. Hydroxyl radicals were produced by pulse radiolysis of water vapour and the decay rate was studied by monitoring the transient light absorption at 3090 A. Arrhenius parameters (A,Esub(a)) for the reaction RH + OH arrow R + HOH were obtained for the reactants RH = C 2 H 6 ,CH 3 Cl,CH 2 Cl 2 ,CHFCl 2 . CF 2 Cl 2 (CFC-12) which contains no C-H bonds was found to be inert toward attack by hydroxyl radicals. Since no other tropospheric sink reactions have been reported for CFC-12 its possible impact on the stratospheric ozone remains a controversial problem. (author)

Thermochemistry and kinetics for 2-butanone-1-yl radical (CH2·C(═O)CH2CH3) reactions with O2.

Science.gov (United States)

Sebbar, N; Bozzelli, J W; Bockhorn, H

2014-01-09

Thermochemistry of reactants, intermediates, transition state structures, and products along with kinetics on the association of CH2·C(═O)CH2CH3 (2-butanone-1-yl) with O2 and dissociation of the peroxy adduct isomers are studied. Thermochemical properties are determined using ab initio (G3MP2B3 and G3) composite methods along with density functional theory (B3LYP/6-311g(d,p)). Entropy and heat capacity contributions versus temperature are determined from structures, vibration frequencies, and internal rotor potentials. The CH2·C(═O)CH2CH3 radical + O2 association results in a chemically activated peroxy radical with 27 kcal mol(-1) excess of energy. The chemically activated adduct can react to stabilized peroxy or hydroperoxide alkyl radical adducts, further react to lactones plus hydroxyl radical, or form olefinic ketones and a hydroperoxy radical. Kinetic parameters are determined from the G3 composite methods derived thermochemical parameters, and quantum Rice-Ramsperger-Kassel (QRRK) analysis to calculate k(E) with master equation analysis to evaluate falloff in the chemically activated and dissociation reactions. One new, not previously reported, peroxy chemistry reaction is presented. It has a low barrier path and involves a concerted reaction resulting in olefin formation, H2O elimination, and an alkoxy radical.

Radical carbonylations using a continuous microflow system

Directory of Open Access Journals (Sweden)

Takahide Fukuyama

2009-07-01

Full Text Available Radical-based carbonylation reactions of alkyl halides were conducted in a microflow reactor under pressurized carbon monoxide gas. Good to excellent yields of carbonylated products were obtained via radical formylation, carbonylative cyclization and three-component coupling reactions, using tributyltin hydride or TTMSS as a radical mediator.

Aerosol Fragmentation Driven by Coupling of Acid-Base and Free-Radical Chemistry in the Heterogeneous Oxidation of Aqueous Citric Acid by OH Radicals.

Science.gov (United States)

Liu, Matthew J; Wiegel, Aaron A; Wilson, Kevin R; Houle, Frances A

2017-08-10

A key uncertainty in the heterogeneous oxidation of carboxylic acids by hydroxyl radicals (OH) in aqueous-phase aerosol is how the free-radical reaction pathways might be altered by acid-base chemistry. In particular, if acid-base reactions occur concurrently with acyloxy radical formation and unimolecular decomposition of alkoxy radicals, there is a possibility that differences in reaction pathways impact the partitioning of organic carbon between the gas and aqueous phases. To examine these questions, a kinetic model is developed for the OH-initiated oxidation of citric acid aerosol at high relative humidity. The reaction scheme, containing both free-radical and acid-base elementary reaction steps with physically validated rate coefficients, accurately predicts the experimentally observed molecular composition, particle size, and average elemental composition of the aerosol upon oxidation. The difference between the two reaction channels centers on the reactivity of carboxylic acid groups. Free-radical reactions mainly add functional groups to the carbon skeleton of neutral citric acid, because carboxylic acid moieties deactivate the unimolecular fragmentation of alkoxy radicals. In contrast, the conjugate carboxylate groups originating from acid-base equilibria activate both acyloxy radical formation and carbon-carbon bond scission of alkoxy radicals, leading to the formation of low molecular weight, highly oxidized products such as oxalic and mesoxalic acid. Subsequent hydration of carbonyl groups in the oxidized products increases the aerosol hygroscopicity and accelerates the substantial water uptake and volume growth that accompany oxidation. These results frame the oxidative lifecycle of atmospheric aerosol: it is governed by feedbacks between reactions that first increase the particle oxidation state, then eventually promote water uptake and acid-base chemistry. When coupled to free-radical reactions, acid-base channels lead to formation of low molecular

Heterogeneous reaction of particulate chlorpyrifos with NO3 radicals: Products, pathways, and kinetics

Science.gov (United States)

Li, Nana; Zhang, Peng; Yang, Bo; Shu, Jinian; Wang, Youfeng; Sun, Wanqi

2014-08-01

Chlorpyrifos is a typical chlorinated organophosphorus pesticide. The heterogeneous reaction of chlorpyrifos particles with NO3 radicals was investigated using a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS) and a real-time atmospheric gas analysis mass spectrometer. Chlorpyrifos oxon, 3,5,6-trichloro-2-pyridinol, O,O-diethyl O-hydrogen phosphorothioate, O,O-diethyl ester thiophosphoric acid, diethyl hydrogen phosphate and a phosphinyl disulfide compound were identified as the main degradation products. The heterogeneous reaction pathways were proposed and their kinetic processes were investigated via a mixed-phase relative rate method. The observed effective rate constant is 3.4 ± 0.2 × 10-12 cm3 molecule-1 s-1.

Effects of hydroxylated benzaldehyde derivatives on radiation-induced reactions involving various organic radicals

Science.gov (United States)

Ksendzova, G. A.; Samovich, S. N.; Sorokin, V. L.; Shadyro, O. I.

2018-05-01

In the present paper, the effects of hydroxylated benzaldehyde derivatives and gossypol - the known natural occurring compound - on formation of decomposition products resulting from radiolysis of ethanol and hexane in deaerated and oxygenated solutions were studied. The obtained data enabled the authors to make conclusions about the effects produced by the structure of the compounds under study on their reactivity towards oxygen- and carbon-centered radicals. It has been found that 2,3-dihydroxybenzaldehyde, 4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde and 4,6-di-tert-butyl-3-(1,3-dioxane-2-yl)-1,2-dihydroxybenzene are not inferior in efficiency to butylated hydroxytoluene - the industrial antioxidant - as regards suppression of the radiation-induced oxidation processes occurring in hexane. The derivatives of hydroxylated benzaldehydes were shown to have a significant influence on radiation-induced reactions involving α-hydroxyalkyl radicals.

Mechanisms for formation of organic acids in gas-phase reactions of ozone and hydroxyl radical with dialkenes and unsaturated carbonyls

Science.gov (United States)

Chien, Chao-Jung

2001-07-01

Carboxylic acids are ubiquitous throughout the troposphere and may contribute significant fractions of the free acidity in some remote areas. One of the important sources of these carboxylic acids is thought to be photochemical transformation of biogenic hydrocarbons such as isoprene. For the work reported here, atmospheric samples from University of North Carolina dual outdoor environmental chamber under simulated urban atmospheric conditions were analyzed for carboxylic acids. Both OH radicals and O3 initiated photooxidation reaction experiments were performed for isoprene, along with its structural analogs, 1,3-butadiene and 2,3-dimethyl-1,3-butadiene, and their primary photooxidation products, methacrolein, acrolein, and methyl vinyl ketone. Among the detected carboxylic acids were formic, acetic, and several multifunctional carboxylic acids, including methacrylic, acrylic, glyoxylic, and glycolic acids. Quantification of most carboxylic acid products was also established. Formation yields of carboxylic acids from the reactions of O3 with studied compounds were determined, and time-concentration series of the reactants and carboxylic acid products were measured to facilitate mechanism formulation. While the reaction mechanisms of Criegee biradicals arising from decomposition of primary ozonides are proposed to account for the observed carboxylic acid products in the ozonolysis of unsaturated hydrocarbons, reactions of peroxy acyl radicals with HO2 and/or other peroxy radicals are thought to be responsible for the formation of carboxylic acids during the OH-initiated reactions in the presence of NOx. In this study, smog chamber simulations have also been performed for selected compounds using Morpho, a photochemical kinetic simulation software package. Explicit photochemical mechanisms with O 3 and OH radicals that lead to formation of carboxylic acids were elaborated and implemented, and the simulation results were compared with those from other chemical

Radical intermediates involved in the bleaching of the carotenoid crocin. Hydroxyl radicals, superoxide anions and hydrated electrons

International Nuclear Information System (INIS)

Bors, W.; Saran, M.; Michel, C.

1982-01-01

The participation of the primary radicals in the bleaching of aqueous solutions of the carotenoid crocin by ionizing radiation was investigated, employing both X-radiolysis and pulse radiolysis. The pulse-radiolytic data demonstrated a very rapid diffusion-controlled attack by both hydroxyl radicals (radicalsOH) and hydrated electrons (e - sub(aq)), while superoxide anions (O 2 - ) did not react at all. The site of the initial reaction of these radicals was not limited to the polyene chromophore. Slower secondary reactions involving crocin alkyl or peroxy radicals contribute mainly to the overall bleaching, in particular during steady-state irradiation. (author)

Visible-Light Photoredox-Catalyzed Giese Reaction: Decarboxylative Addition of Amino Acid Derived α-Amino Radicals to Electron-Deficient Olefins

KAUST Repository

Millet, Anthony

2016-06-20

A tin- and halide-free, visible-light photoredox-catalyzed Giese reaction was developed. Primary and secondary α-amino radicals were generated readily from amino acids in the presence of catalytic amounts of an iridium photocatalyst. The reactivity of the α-amino radicals has been evaluated for the functionalization of a variety of activated olefins. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Visible-Light Photoredox-Catalyzed Giese Reaction: Decarboxylative Addition of Amino Acid Derived α-Amino Radicals to Electron-Deficient Olefins

KAUST Repository

Millet, Anthony; Lefebvre, Quentin; Rueping, Magnus

2016-01-01

A tin- and halide-free, visible-light photoredox-catalyzed Giese reaction was developed. Primary and secondary α-amino radicals were generated readily from amino acids in the presence of catalytic amounts of an iridium photocatalyst. The reactivity of the α-amino radicals has been evaluated for the functionalization of a variety of activated olefins. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Investigation of the O+allyl addition/elimination reaction pathways from the OCH2CHCH2 radical intermediate

International Nuclear Information System (INIS)

FitzPatrick, Benjamin L.; Lau, K.-C.; Butler, Laurie J.; Lee, S.-H.; Lin, Jim Jr-Min

2008-01-01

These experiments study the preparation of and product channels resulting from OCH 2 CHCH 2 , a key radical intermediate in the O+allyl bimolecular reaction. The data include velocity map imaging and molecular beam scattering results to probe the photolytic generation of the radical intermediate and the subsequent pathways by which the radicals access the energetically allowed product channels of the bimolecular reaction. The photodissociation of epichlorohydrin at 193.3 nm produces chlorine atoms and c-OCH 2 CHCH 2 radicals; these undergo a facile ring opening to the OCH 2 CHCH 2 radical intermediate. State-selective resonance-enhanced multiphoton ionization (REMPI) detection resolves the velocity distributions of ground and spin-orbit excited state chlorine independently, allowing for a more accurate determination of the internal energy distribution of the nascent radicals. We obtain good agreement detecting the velocity distributions of the Cl atoms with REMPI, vacuum ultraviolet (VUV) photoionization at 13.8 eV, and electron bombardment ionization; all show a bimodal distribution of recoil kinetic energies. The dominant high recoil kinetic energy feature peaks near 33 kcal/mol. To elucidate the product channels resulting from the OCH 2 CHCH 2 radical intermediate, the crossed laser-molecular beam experiment uses VUV photoionization and detects the velocity distribution of the possible products. The data identify the three dominant product channels as C 3 H 4 O (acrolein)+H, C 2 H 4 +HCO (formyl radical), and H 2 CO (formaldehyde)+C 2 H 3 . A small signal from C 2 H 2 O (ketene) product is also detected. The measured velocity distributions and relative signal intensities at m/e=27, 28, and 29 at two photoionization energies show that the most exothermic product channel, C 2 H 5 +CO, does not contribute significantly to the product branching. The higher internal energy onset of the acrolein+H product channel is consistent with the relative barriers en route to

Investigation of the O+allyl addition/elimination reaction pathways from the OCH2CHCH2 radical intermediate

Science.gov (United States)

Fitzpatrick, Benjamin L.; Lau, Kai-Chung; Butler, Laurie J.; Lee, Shih-Huang; Lin, Jim-Min, Jr.

2008-08-01

These experiments study the preparation of and product channels resulting from OCH2CHCH2, a key radical intermediate in the O+allyl bimolecular reaction. The data include velocity map imaging and molecular beam scattering results to probe the photolytic generation of the radical intermediate and the subsequent pathways by which the radicals access the energetically allowed product channels of the bimolecular reaction. The photodissociation of epichlorohydrin at 193.3 nm produces chlorine atoms and c-OCH2CHCH2 radicals; these undergo a facile ring opening to the OCH2CHCH2 radical intermediate. State-selective resonance-enhanced multiphoton ionization (REMPI) detection resolves the velocity distributions of ground and spin-orbit excited state chlorine independently, allowing for a more accurate determination of the internal energy distribution of the nascent radicals. We obtain good agreement detecting the velocity distributions of the Cl atoms with REMPI, vacuum ultraviolet (VUV) photoionization at 13.8 eV, and electron bombardment ionization; all show a bimodal distribution of recoil kinetic energies. The dominant high recoil kinetic energy feature peaks near 33 kcal/mol. To elucidate the product channels resulting from the OCH2CHCH2 radical intermediate, the crossed laser-molecular beam experiment uses VUV photoionization and detects the velocity distribution of the possible products. The data identify the three dominant product channels as C3H4O (acrolein)+H, C2H4+HCO (formyl radical), and H2CO (formaldehyde)+C2H3. A small signal from C2H2O (ketene) product is also detected. The measured velocity distributions and relative signal intensities at m/e=27, 28, and 29 at two photoionization energies show that the most exothermic product channel, C2H5+CO, does not contribute significantly to the product branching. The higher internal energy onset of the acrolein+H product channel is consistent with the relative barriers en route to each of these product channels

Fragmentation characteristics of the unstable [CH3 CO][radical sign] radicals generated by neutralization of [CH3CO]+ cations

Science.gov (United States)

Hop, C. E. C. A.; Holmes, J. L.

1991-03-01

The stability and fragmentation characteristics of [CH3 CO][radical sign] radicals, generated by vertical charge exchange between acetyl cations and permanent gases or metal vapours (He, Xe, NO, Cd, Na and K), were examined mass spectrometrically. Two dissociation reactions were observed, the losses of CH[radical sign]3 and H[radical sign]. The H[radical sign] loss reaction, the higher energy dissociation, became of greater importance as the exothermicity of the charge exchange was increased. Based on the analysis of the kinetic energy releases it was concluded that these decompositions arose from the population of two excited states of the [CH3 CO][radical sign] radical.

Enantioselective synthesis of tetrafluorinated ribose and fructose.

Science.gov (United States)

Linclau, Bruno; Boydell, A James; Timofte, Roxana S; Brown, Kylie J; Vinader, Victoria; Weymouth-Wilson, Alexander C

2009-02-21

A perfluoroalkylidene lithium mediated cyclisation approach for the enantioselective synthesis of a tetrafluorinated aldose (ribose) and of a tetrafluorinated ketose (fructose), both in the furanose and in the pyranose form, is described.

Temperature dependence of carbon kinetic isotope effect for the oxidation reaction of ethane by OH radicals under atmospherically relevant conditions

Science.gov (United States)

Piansawan, Tammarat; Saccon, Marina; Laumer, Werner; Gensch, Iulia; Kiendler-Scharr, Astrid

2015-04-01

Modeling of the global distribution of atmospheric ethane sources and sinks by using the 13C isotopic composition requires accurate knowledge of the carbon kinetic isotope effect (KIE) of its atmospheric removal reactions. The quantum mechanical prediction implies the necessity to elucidate the temperature dependence of KIE within atmospherically relevant temperature range by experiment. In this study, the KIE and its temperature dependence for ethane oxidation by OH radicals was investigated at ambient pressure in a temperature range of 243 K to 303 K. The chemical reactions were carried out in a 15 L PFE reaction chamber, suspended in a thermally controlled oven. The isotope ratios of the gas phase components during the course of the reactions were measured by Thermal Desorption -- Gas Chromatography -- Isotope Ratio Mass Spectrometry (TD-GC-IRMS). For each temperature, the KIE was derived from the temporal evolution of the concentration and stable carbon isotope ratio (δ13C) of ethane using a method adapted from the relative reaction rate concept. The room temperature KIE of the ethane reaction with OH radicals was found to be 6.85 ± 0.32 ‰. This value is in agreement with the previously reported value of 8.57 ± 1.95 ‰ [Anderson et al. 2004] but has a substantially lower uncertainty. The experimental results will be discussed with the KIE temperature dependence predicted by quantum mechanical calculations. Reference: Rebecca S. Anderson, Lin Huang, Richard Iannone, Alexandra E. Thompson, and Jochen Rudolph (2004), Carbon Kinetic Isotope Effects in the Gas Phase Reactions of Light Alkanes and Ethene with the OH Radical at 296 ± 4 K, J. Phys. Chem. A, 108, 11537--11544

Reactions of OH Radicals with Tris (1,10-Phenanthroline) Iron (II) Studied by Pulse Radiolysis

DEFF Research Database (Denmark)

Siekierska Floryan, E.; Pagsberg, Palle Bjørn

1976-01-01

The reaction of OH radicals with aqueous tris(1,10-phenanthroline)iron(II) leads to the formation of an adduct, which exhibits a broad absorption band at rmpH = 6, λmax = 460 nm, and epsilon (Porson)460 = 6700 (molar, decadic, 1 mol−1 cm−1). The rate of formation of the adduct is first order...... in complex concentration with a bimolecular rate constant Image independent of pH in the range pH 3–11. The adduct decays by mixed-order kinetics, but at 310 nm a second-order formation of a decay product can be directly observed. The reaction of OH radicals with aqueous 1,10-phenanthroline leads also...... to the formation of an adduct which absorbs in the whole visible region with a maximum at 425 nm and ε425 = 2612 (molar, decadic, 1 mol−1 cm−1) in neutral solution. The adduct exhibits a red shift in acidic and alkaline media. The formation is first order in 1,10-phenanthroline with a bimolecular rate constant...

Rate constants for the reaction of CF3O radicals with hydrocarbons at 298 K

DEFF Research Database (Denmark)

Kelly, C.; Treacy, J.; Sidebottom, H.W.

1993-01-01

Rate constant ratios of the reactions of CF3O radicals with a number of hydrocarbons have been determined at 298 +/- 2 K and atmospheric pressure using a relative rate method. Using a previously determined value k(CF30 + C2H6) = 1.2 x 10(-12) cm3 molecule-1 s-1 these rate constant ratios provide...... estimates of the rate constants: k(CF3O + CH4) = (1.2 +/- 0.1) x 10(-14), k(CF3O + c-C3H6) = (3.6 +/- 0.2) x 10(-13), k(CF3O + C3H8) = (4.7 +/- 0.7) x 10(-12), k(CF3O + (CH3)3CH) = (7.2 +/- 0.5) x 10(-12), k(CF3O + C2H4) = (3.0 +/- 0.1) x 10(-11) and k(CF3O + C6H6) = (3.6 +/- 0.1) x 10(-11) cm3 molecule-1 s......-1. The importance of the reactions of CF3O radicals with hydrocarbons under atmospheric conditions is discussed....

Reactions of inorganic free radicals with liver protecting drugs

International Nuclear Information System (INIS)

Gyoergy, I.; Foeldiak, G.; Blazovics, A.; Feher, J.

1990-01-01

Liver protecting drugs, silibinin, a flavonolignane, and the dihydroquinoline derivatives, CH 402 and MTDQ-DA, were shown to inhibit processes in which enzymatically or non-enzymatically generated free radicals were involved. Inorganic free radicals (N 3 , (SCN) 2 - , OH, Trp, CO 2 - , O 2 - ) produced by pulse radiolysis readily react with the compounds, which transform into exceptionally long-lived, unreactive transients. Time evolution of the UV and visible spectra indicate that oxidising radicals form a phenoxyl type radical from silibinin, while OH forms an adduct by attacking, simultaneously, at various sites of the molecule. Superoxide radicals reduce silibinin and oxidise CH 402 and MTDQ-DA. It is concluded that the drugs might exhibit antioxidant behavior in living systems. (author)

Asymmetric Radical Cyclopropanation of Alkenes with In Situ-Generated Donor-Substituted Diazo Reagents via Co(II)-Based Metalloradical Catalysis.

Science.gov (United States)

Wang, Yong; Wen, Xin; Cui, Xin; Wojtas, Lukasz; Zhang, X Peter

2017-01-25

Donor-substituted diazo reagents, generated in situ from sulfonyl hydrazones in the presence of base, can serve as suitable radical precursors for Co(II)-based metalloradical catalysis (MRC). The cobalt(II) complex of D 2 -symmetric chiral porphyrin [Co(3,5-Di t Bu-Xu(2'-Naph)Phyrin)] is an efficient metalloradical catalyst that is capable of activating different N-arylsulfonyl hydrazones for asymmetric radical cyclopropanation of a broad range of alkenes, affording the corresponding cyclopropanes in high yields with effective control of both diastereo- and enantioselectivity. This Co(II)-based metalloradical system represents the first catalytic protocol that can effectively utilize donor-type diazo reagents for asymmetric olefin cyclopropanation.

Kinetic investigation on enantioselective hydrolytic resolution of ...

African Journals Online (AJOL)

Kinetic investigation on enantioselective hydrolytic resolution of epichlorohydrin by crude epoxide hydrolase from domestic duck liver. X Ling, D Lu, J Wang, J Chen, L Ding, J Chen, H Chai, P Ouyang ...

Asymmetric Diels-Alder Reaction of α-Substituted and β,β-Disubstituted α,β-Enals via Diarylprolinol Silyl Ether for the Construction of All-Carbon Quaternary Stereocenters.

Science.gov (United States)

Hayashi, Yujiro; Bondzic, Bojan P; Yamazaki, Tatsuya; Gupta, Yogesh; Ogasawara, Shin; Taniguchi, Tohru; Monde, Kenji

2016-10-24

The asymmetric Diels-Alder reaction of α-substituted acrolein proceeds in the presence of the trifluoroacetic acid salt of trifluoromethyl-substituted diarylprolinol silyl ether to afford the exo-isomer with both excellent diastereoselectivity and high enantioselectivity. In the Diels-Alder reaction of a β,β-disubstituted α,β-unsaturated aldehyde, good exo-selectivity and excellent enantioselectivity was obtained when the perchloric acid salt of the bulky triisopropyl silyl ether of trifluoromethyl substituted diarylprolinol was employed as an organocatalyst in the presence of water. In both cases, all-carbon quaternary stereocenters are constructed enantioselectively. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The Nanoconfined Free Radical Polymerization: Reaction Kinetics and Thermodynamics

Science.gov (United States)

Zhao, Haoyu; Simon, Sindee

The reaction kinetics and thermodynamics of nanoconfined free radical polymerizations are investigated for methyl methacrylate (MMA) and ethyl methacrylate (EMA) monomers using differential scanning calorimetry. Controlled pore glass is used as the confinement medium with pore diameters as small as 7.5 nm; the influence of both hydrophobic (silanized such that trimethylsilyl groups cover the surface) and hydrophilic (native silanol) surfaces is investigated. Propagation rates increase when monomers are reacted in the hydrophilic pores presumably due to the specific interactions between the carbonyl and silanol groups; however, the more flexible EMA monomer shows weaker effects. On the other hand, initial rates of polymerization in hydrophobic pores are unchanged from the bulk. In both pores, the onset of autoacceleration occurs earlier due to the reduced diffusivity of confined chains, which may be compensated at high temperatures. In addition to changes in kinetics, the reaction thermodynamics can be affected under nanoconfinement. Specifically, the ceiling temperature (Tc) is shifted to lower temperatures in nanopores, with pore surface chemistry showing no significant effects; the equilibrium conversion is also reduced at high temperatures below Tc. These observations are attributed to a larger negative change in entropy on propagation for the confined system, with the MMA system again showing greater effects. Funding from ACS PRF is gratefully acknowledged.

Glutathione as a radical scavenger and the biological consequences of thiyl radical production

International Nuclear Information System (INIS)

Winterbourn, C.C.

1996-01-01

A large number of compounds that have toxic effects can be metabolised to free radicals and secondary reactive oxygen species. These may be directly damaging or affect cell function by altering regulatory mechanisms through changing redox status. Protection is provided by an integrated system of antioxidant defenses. This includes reduced glutathione (GSH), one of the functions of which is as a free radical scavenger. For GSH to be an effective radical scavenging antioxidant, therefore, it must act in concert with superoxide dismutase to remove the superoxide so generated. Superoxide is produced in a variety of metabolic processes. It is also a secondary product of radicals reacting with oxygen either directly or through GSH. The biological reactivity of superoxide has been the subject of much debate ever since the discovery of superoxide dismutase in 1968. It has more recently become apparent that its rapid reaction with nitric oxide to give peroxynitrite, and its ability to reversibly oxidise and inactivate iron sulphur enzymes, contribute to the toxicity of superoxide. Another mechanism that could be important involves addition reactions of superoxide with other radicals to give organic peroxides. This reaction, to form a tyrosine peroxide, has come to authors attention through the study of the scavenging of tyrosyl radicals by GSH. It is also shown that a tyrosine peroxide is a major product of the oxidation of tyrosine by neutrophils

Bifunctional nanocrystalline MgO for chiral epoxy ketones via Claisen-Schmidt condensation-asymmetric epoxidation reactions.

Science.gov (United States)

Choudary, Boyapati M; Kantam, Mannepalli L; Ranganath, Kalluri V S; Mahendar, Koosam; Sreedhar, Bojja

2004-03-24

Design and development of a truly nanobifunctional heterogeneous catalyst for the Claisen-Schmidt condensation (CSC) of benzaldehydes with acetophenones to yield chalcones quantitatively followed by asymmetric epoxidation (AE) to afford chiral epoxy ketones with moderate to good yields and impressive ee's is described. The nanomagnesium oxide (aerogel prepared) NAP-MgO was found to be superior over the NA-MgO and CM-MgO in terms of activity and enantioselectivity as applicable in these reactions. An elegant strategy for heterogenization of homogeneous catalysts is presented here to evolve single-site chiral catalysts for AE by a successful transfer of molecular chemistry to surface metal-organic chemistry with the retention of activity, selectivity/enantioselectivity. Brønsted hydroxyls are established as sole contributors for the epoxidation reaction, while they add on to the CSC, which is largely driven by Lewis basic O2-sites. Strong hydrogen-bond interactions between the surface -OH on MgO and -OH groups of diethyl tartrate are found inducing enantioselectivity in the AE reaction. Thus, the nanocrystalline NAP-MgO with its defined shape, size, and accessible OH groups allows the chemisorption of TBHP, DET, and olefin on its surface to accomplish single-site chiral catalysts to provide optimum ee's in AE reactions.

Asymmetric Brønsted acid-catalyzed aza-Diels–Alder reaction of cyclic C-acylimines with cyclopentadiene

Directory of Open Access Journals (Sweden)

Magnus Rueping

2012-10-01

Full Text Available A new chiral Brønsted acid-catalyzed aza-Diels–Alder reaction of cyclic C-acylimines with cyclopentadiene has been developed. The reaction provides optically active aza-tetracycles in good yields with high diastereo- and enantioselectivities under mild reaction conditions.

Asymmetrical distorted structure, dynamics, and reactions of the silacyclohexane and related radical cations: ESR and ab-initio MO study

International Nuclear Information System (INIS)

Komaguchi, Kenji; Shiotani, Masaru; Ishikawa, Mitsuo

1995-01-01

The σ-type radical cations generated by one electron oxidation of the saturated hydrocarbon have been attracted much attention because of their fundamental importance as primary reactant species in radiation chemistry. Our studies on σ-type radical cations were recently extended to the silacyclohexane (cSiC5), silacyclopentane (cSiC4), and silacyclobutane (cSiC3) radical cations. Their electronic structure, dynamics, and reactions were investigated by means of low temperature matrix isolation ESR technique combined with ionizing radiation (γ-rays from 60 Co). In the preceding paper, the 1-methylsilacyclohexane (1-Me-cSiC5) radical cation has been found to take an asymmetrically distorted C 1 structure with one of two Si-C bonds elongated in which the unpaired electron mainly resides ( 2 A in C 1 ). This conclusion was based on the 4.2 K ESR spectra of radical cations of selectively deuteriated and/or methylsubstituted silacyclohexanes, i.e., cSiC5-2,2,6,6-d 4 + , 1-Me-cSiC5 + , 1-Me-cSiC5-2,2-d 2 + , 1-Me-cSiC5-2,2,6,6-d 4 + , 1,1-Me 2 -cSiC5 + , and 4,4-Me 2 -cSiC5 + , in a frozen CF 3 -cC 6 F 11 matrix. Here we report further experimental and theoretical results on 1-methylsilacyclohexane radical cation, especially on the ab initio MO results and matrix effects on the structural distortion, as well as thermal reactions of the radical cations. The results will make it clear that the distorted C 1 structure of the 1-Me-cSiC5 + is the intrinsic nature at the ground electronic state. (J.P.N.)

Iptycenes with an acridinone motif developed through [4+2] cycloaddition of tethered naphthalene and iminoquinone via a radical reaction.

Science.gov (United States)

Raju, Selvam; Annamalai, Pratheepkumar; Chen, Pei-Ling; Liu, Yi-Hung; Chuang, Shih-Ching

2017-06-06

A new class of iptycenes was developed by combining 2-(naphthalen-1-yl)anilines and p-benzoquinones through copper(ii)-mediated radical cyclisation. This unusual cyclisation reaction resulted in the robust and efficient syntheses of iptycenes with an acridinone motif. These iptycenes can be further transformed into planar acridinone heterocyclics through the Diels-Alder reaction.

Serum Hydroxyl Radical Scavenging Capacity as Quantified with Iron-Free Hydroxyl Radical Source

Science.gov (United States)

Endo, Nobuyuki; Oowada, Shigeru; Sueishi, Yoshimi; Shimmei, Masashi; Makino, Keisuke; Fujii, Hirotada; Kotake, Yashige

2009-01-01

We have developed a simple ESR spin trapping based method for hydroxyl (OH) radical scavenging-capacity determination, using iron-free OH radical source. Instead of the widely used Fenton reaction, a short (typically 5 seconds) in situ UV-photolysis of a dilute hydrogen peroxide aqueous solution was employed to generate reproducible amounts of OH radicals. ESR spin trapping was applied to quantify OH radicals; the decrease in the OH radical level due to the specimen’s scavenging activity was converted into the OH radical scavenging capacity (rate). The validity of the method was confirmed in pure antioxidants, and the agreement with the previous data was satisfactory. In the second half of this work, the new method was applied to the sera of chronic renal failure (CRF) patients. We show for the first time that after hemodialysis, OH radical scavenging capacity of the CRF serum was restored to the level of healthy control. This method is simple and rapid, and the low concentration hydrogen peroxide is the only chemical added to the system, that could eliminate the complexity of iron-involved Fenton reactions or the use of the pulse-radiolysis system. PMID:19794928

Radical chemistry of artemisinin

Energy Technology Data Exchange (ETDEWEB)

Denisov, Evgenii T; Solodova, S L; Denisova, Taisa G [Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region (Russian Federation)

2011-12-29

The review summarizes physicochemical characteristics of the natural sesquiterpene peroxide artemisinin. The kinetic schemes of transformations of artemisinin radicals under anaerobic conditions are presented and analyzed. The sequence of radical reactions of artemisinin in the presence of oxygen is considered in detail. Special emphasis is given to the intramolecular chain oxidation resulting in the transformation of artemisinin into polyatomic hydroperoxide. The kinetic characteristics of elementary reaction steps involving alkyl, alkoxyl, and peroxyl radicals generated from artemisinin are discussed. The results of testing of artemisinin and its derivatives for the antimalarial activity and the scheme of the biochemical synthesis of artemisinin in nature are considered.

Radical chemistry of artemisinin

Science.gov (United States)

Denisov, Evgenii T.; Solodova, S. L.; Denisova, Taisa G.

2010-12-01

The review summarizes physicochemical characteristics of the natural sesquiterpene peroxide artemisinin. The kinetic schemes of transformations of artemisinin radicals under anaerobic conditions are presented and analyzed. The sequence of radical reactions of artemisinin in the presence of oxygen is considered in detail. Special emphasis is given to the intramolecular chain oxidation resulting in the transformation of artemisinin into polyatomic hydroperoxide. The kinetic characteristics of elementary reaction steps involving alkyl, alkoxyl, and peroxyl radicals generated from artemisinin are discussed. The results of testing of artemisinin and its derivatives for the antimalarial activity and the scheme of the biochemical synthesis of artemisinin in nature are considered.

Radical chemistry of artemisinin

International Nuclear Information System (INIS)

Denisov, Evgenii T; Solodova, S L; Denisova, Taisa G

2010-01-01

The review summarizes physicochemical characteristics of the natural sesquiterpene peroxide artemisinin. The kinetic schemes of transformations of artemisinin radicals under anaerobic conditions are presented and analyzed. The sequence of radical reactions of artemisinin in the presence of oxygen is considered in detail. Special emphasis is given to the intramolecular chain oxidation resulting in the transformation of artemisinin into polyatomic hydroperoxide. The kinetic characteristics of elementary reaction steps involving alkyl, alkoxyl, and peroxyl radicals generated from artemisinin are discussed. The results of testing of artemisinin and its derivatives for the antimalarial activity and the scheme of the biochemical synthesis of artemisinin in nature are considered.

Radical chemistry of artemisinin

Energy Technology Data Exchange (ETDEWEB)

Denisov, Evgenii T; Solodova, S L; Denisova, Taisa G [Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region (Russian Federation)

2010-12-29

The review summarizes physicochemical characteristics of the natural sesquiterpene peroxide artemisinin. The kinetic schemes of transformations of artemisinin radicals under anaerobic conditions are presented and analyzed. The sequence of radical reactions of artemisinin in the presence of oxygen is considered in detail. Special emphasis is given to the intramolecular chain oxidation resulting in the transformation of artemisinin into polyatomic hydroperoxide. The kinetic characteristics of elementary reaction steps involving alkyl, alkoxyl, and peroxyl radicals generated from artemisinin are discussed. The results of testing of artemisinin and its derivatives for the antimalarial activity and the scheme of the biochemical synthesis of artemisinin in nature are considered.

Enantiomerization and enantioselective bioaccumulation of metalaxyl in Tenebrio molitor larvae.

Science.gov (United States)

Gao, Yongxin; Wang, Huili; Qin, Fang; Xu, Peng; Lv, Xiaotian; Li, Jianzhong; Guo, Baoyuan

2014-02-01

The enantiomerization and enantioselective bioaccumulation of metalaxyl by a single dose of exposure to Tenebrio molitor larvae under laboratory condition were studied by high-performance liquid chromatography tandem mass spectroscopy (HPLC-MS/MS) based on a ChiralcelOD-3R [cellulosetris-tris-(3, 5-dichlorophenyl-carbamate)] column. Exposure of enantiopure R-metalaxyl and S-metalaxyl in Tenebrio molitor larvae exhibited significant enantiomerization, with formation of the R enantiomers from the S enantiomers, and vice versa, which might be attributed to the chiral pesticide catalyzed by a certain enzyme in Tenebrio molitor larvae. Enantiomerization was not observed in wheat bran during the period of 21 d. In addition, bioaccumulation of rac-metalaxyl in Tenebrio molitor larvae was enantioselective with a preferential accumulation of S-metalaxyl. These results showed that enantioselectivity was caused not only by actual degradation and metabolism but also by enantiomerization, which was an important process in the environmental fate and behavior of metalaxyl enantiomers. Copyright © 2013 Wiley Periodicals, Inc.

Potencial de biocatálise enantiosseletiva de lipases microbianas Potential of enantioselective biocatalysis by microbial lipases

Directory of Open Access Journals (Sweden)

Patrícia de O. Carvalho

2005-08-01

Full Text Available Microbial lipases have a great potential for commercial applications due to their stability, selectivity and broad substrate specificity because many non-natural acids, alcohols or amines can be used as the substrate. Three microbial lipases isolated from Brazilian soil samples (Aspergillus niger; Geotrichum candidum; Penicillium solitum were compared in terms of their stability and as biocatalysts in the enantioselective esterification using racemic substrates in organic medium. The lipase from Aspergillus niger showed the highest activity (18.2 U/mL and was highly thermostable, retaining 90% and 60% activity at 50 ºC and 60 ºC after 1 hour, respectively. In organic medium, this lipase provided the best results in terms of enantiomeric excess of the (S-active acid (ee = 6.1% and conversion value (c = 20% in the esterification of (R,S-ibuprofen with 1-propanol in isooctane. The esterification reaction of the racemic mixture of (R,S-2-octanol with decanoic acid proceeded with high enantioselectivity when lipase from Aspergillus niger (E = 13.2 and commercial lipase from Candida antarctica (E = 20 were employed.

ESR of spin-trapped radicals from sugars. Reactions of hydroxyl radicals in aqueous solutions and. gamma. -radiolysis in the polycrystalline state

Energy Technology Data Exchange (ETDEWEB)

Kuwabara, M; Lion, Y; Riesz, P [National Cancer Inst., Bethesda, MD (USA)

1981-04-01

For glucose, mannose, and lactose which contain a -CH/sub 2/OH group at the C(5') position, a large secondary doublet (4.8 to 5.6 G) was observed for both solid state and aqueous experiments. However, for fructose, D-deoxyribose and D-ribose, which do not have a -CH/sub 2/OH group at the C(5') position, the large doublet was not found. The small doublet (1.2 to 1.5 G) was obtained from all the sugars for both the solid and the aqueous experiments. The 2.5 to 3.0 G doublet was only observed in the experiments with -irradiated solids, while the 4.3 G doublet was only produced by the reaction of OH radicals. It was inferred that the large (4.8 to 5.6 G) doublet can be assigned to the H-abstraction radical, -CHOH, from the -CH/sub 2/OH group at the C(5') position.

Reaction studies of hot silicon and germanium radicals. Progress report, February 1, 1982-July 31, 1984

International Nuclear Information System (INIS)

Gaspar, P.P.

1984-01-01

The experimental approach toward attaining the goals of this research program is briefly outlined, and the progress made in the 1982 to 1984 period is reviewed in sections entitled: (1) Recoil atom experiments, (2) Studies of thermally and photochemically generated silicon and germanium radicals, and (3) Ion-molecule reaction studies

Free-radical-mediated conjugate additions. Enantioselective synthesis of butyrolactone natural products: (-)-enterolactone, (-)-arctigenin, (-)-isoarctigenin, (-)-nephrosteranic acid, and (-)-roccellaric acid.

Science.gov (United States)

Sibi, Mukund P; Liu, Pingrong; Ji, Jianguo; Hajra, Saumen; Chen, Jian-xie

2002-03-22

Lewis acid-mediated conjugate addition of alkyl radicals to a differentially protected fumarate 10 produced the monoalkylated succinates with high chemical efficiency and excellent stereoselectivity. A subsequent alkylation or an aldol reaction furnished the disubstituted succinates with syn configuration. The chiral auxiliary, 4-diphenylmethyl-2-oxazolidinone, controlled the stereoselectivity in both steps. Manipulation of the disubstituted succinates obtained by alkylation furnished the natural products (-)-enterolactone, (-)-arctigenin, and (-)-isoarctigenin. The overall yields for the target natural products were 20-26% over six steps. Selective functionalization of the disubstituted succinates obtained by aldol condensation gave the paraconic acid natural products (-)-nephrosteranic acid (8) and (-)-roccellaric acid (9). The overall yield of the natural products 8 and 9 over four steps was 53% and 42%, respectively.

Experimental studies of the kinetics of small polyatomic free radicals in combustion reactions; Moniatomisten radikaalien kinetiikka palamisreaktioissa, kokeellinen tutkimus

Energy Technology Data Exchange (ETDEWEB)

Seetula, J. [Helsinki Univ. (Finland). Lab. of Physical Chemistry

1996-12-01

The kinetics of the reactions of CH{sub 2}Cl, CHClBr, CH{sub 3}CCl{sub 2} and CCl{sub 3}, with Cl{sub 2} has been investigated in a tubular reactor coupled to a photoionization mass spectrometer. The radicals of interest, R, were generated homogeneously in the reactor by pulse 248 nm exciplex laser photolysis. The decay of R was monitored as a function of Cl{sub 2} concentration under pseudo-first-order condition to determine the rate constant as a function of temperature pressure. The reactions were studied separately over a temperature range up to a temperature of 873 K. The rate constants of CH{sub 2}Cl, CHClBr and CH{sub 3}CCl{sub 2} radicals determined were fitted to three-parameter Arrhenius-type expression (with units of cm{sup 3} molecule{sup -1} s{sup -1}): k(CH{sub 2}Cl) = 1.05x10{sup -16} x (T){sup 1.4} x exp(-357 J mol{sup -1} / RT), k(CHClBr) = 5.83x10{sup -20} x (T){sup 2.3} x exp(-300 J mol{sup -1}/ RT) and k(CH{sub 3}CCl{sub 2}) 1.10x10{sup -}2{sup 6} x (T){sup 4.3} x exp(15000 J mol{sup -1}/ RT). The rate constants CCl{sub 3} radical were fitted to a two-parameter Arrhenius expression (units in cm{sup 3} molecule{sup -1} s{sup -1}): k(CCl{sub 3}) = (8.1 +- 6.7)10{sup -l3} exp[-(25.0 +- 8.7) kJ mol{sup -1}/ RT]. An Arrhenius expression for the reaction of Cl+CCl{sub 4} -> <- CCl{sub 3}+Cl{sub 2} is also obtained from the kinetics of the reaction of CCl{sub 3} radical with Cl{sub 2} combined with the known heat of formation and entropy values of CCl{sub 3} free radical to be as follows (in units cm{sup 3} molecule{sup -1-}5 s{sup -1}): k(Cl+CCl{sub 4}) = (3.9 + 3.2)10{sup -10} exp[-(71 + 9) kJ mol{sup -1}/ RT]. The error limits stated are l{sigma}+Student`s t and base on statistical uncertainties only. (author)

Chemical repair activity of free radical scavenger edaravone. Reduction reactions with dGMP hydroxyl radical adducts and suppression of base lesions and AP sites on irradiated plasmid DNA

International Nuclear Information System (INIS)

Hata, Kuniki; Katsumura, Yosuke; Urushibara, Ayumi; Yamashita, Shinichi; Lin Mingzhang; Muroya, Yusa; Shikazono, Naoya; Yokoya, Akinari; Fu Haiying

2015-01-01

Reactions of edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) with deoxyguanosine monophosphate (dGMP) hydroxyl radical adducts were investigated by pulse radiolysis technique. Edaravone was found to reduce the dGMP hydroxyl radical adducts through electron transfer reactions. The rate constants of the reactions were greater than 4 × 10 8 dm 3 mol -1 s -1 and similar to those of the reactions of ascorbic acid, which is a representative antioxidant. Yields of single-strand breaks, base lesions, and abasic sites produced in pUC18 plasmid DNA by gamma ray irradiation in the presence of low concentrations (10–1000 μmol dm -3 ) of edaravone were also quantified, and the chemical repair activity of edaravone was estimated by a method recently developed by the authors. By comparing suppression efficiencies to the induction of each DNA lesion, it was found that base lesions and abasic sites were suppressed by the chemical repair activity of edaravone, although the suppression of single-strand breaks was not very effective. This phenomenon was attributed to the chemical repair activity of edaravone toward base lesions and abasic sites. However, the chemical repair activity of edaravone for base lesions was lower than that of ascorbic acid. (author)

Arenethiolatecopper(I) complexes as homogeneous catalysts for Michael addition reactions

NARCIS (Netherlands)

Koten, G. van; Klaveren, M. van; Lambert, F.; Eijkelkamp, D.J.F.M.; Grove, D.M.

1994-01-01

Arenethiolatocopper(I) complexes are shown to be efficient homogeneous catalysts in Michael addition reactions of several Grignard reagents to acyclic enones; the addition products are formed with excellent chemoselectivity (>99%) and good enantioselectivity (76% e.e.).

Concerted effects in the reaction of ·OH radicals with aromatics: radiolytic oxidation of salicylic acid

International Nuclear Information System (INIS)

Albarran, G.; Schuler, R.H.

2003-01-01

Liquid chromatographic and capillary electrophoretic studies have been used to resolve the products produced in the radiolytic oxidation of salicylic acid in aqueous solution. These studies have shown that, as in the case of phenol, · OH radicals preferentially add to the positions ortho and para to the OH substituent. However, in contrast to its reaction with phenol, addition at the ortho position is favored over addition at the para position. Because · OH radical is a strong electrophile this difference suggests that the electron population at the ortho position in the salicylate anion is enhanced as a result of the hydrogen bonding in salicylic acid

Cyclic Versus Linear Isomers Produced by Reaction of the Methylidyne Radical (CH) with Small Unsaturated Hydrocarbons

Energy Technology Data Exchange (ETDEWEB)

Goulay, Fabien; Trevitt, Adam J.; Meloni, Giovanni; Selby, Talitha M.; Osborn, David L.; Taatjes, Craig A.; Vereecken, Luc; Leone, Stephen R.

2008-12-05

The reactions of the methylidyne radical (CH) with ethylene, acetylene, allene, and methylacetylene are studied at room temperature using tunable vacuum ultraviolet (VUV) photoionization and time-resolved mass spectrometry. The CH radicals are prepared by 248 nm multiphoton photolysis of CHBr3 at 298 K and react with the selected hydrocarbon in a helium gas flow. Analysis of photoionization efficiency versus VUV photon wavelength permits isomer-specific detection of the reaction products and allows estimation of the reaction product branching ratios. The reactions proceed by either CH insertion or addition followed by H atom elimination from the intermediate adduct. In the CH + C2H4 reaction the C3H5 intermediate decays by H atom loss to yield 70(+-8)percent allene, 30(+-8)percent methylacetylene and less than 10percent cyclopropene, in agreement with previous RRKM results. In the CH + acetylene reaction, detection of mainly the cyclic C3H2 isomer is contrary to a previous RRKM calculation that predicted linear triplet propargylene to be 90percent of the total H-atom co-products. High-level CBS-APNO quantum calculations and RRKM calculation for the CH + C2H2 reaction presented in this manuscript predict a higher contribution of the cyclic C3H2 (27.0percent) versus triplet propargylene (63.5percent) than these earlier predictions. Extensive calculations on the C3H3 and C3H2D system combined with experimental isotope ratios for the CD + C2H2 reaction indicate that H-atom assisted isomerization in the present experiments is responsible for the discrepancy between the RRKM calculations and the experimental results. Cyclic isomers are also found to represent 30(+-6)percent of the detected products in the case of CH + methylacetylene, together with 33(+-6)percent 1,2,3-butatriene and 37(+-6)percent vinylacetylene. The CH + allene reaction gives 23(+-5)percent 1,2,3-butatriene and 77(+-5)percent vinylacetylene, whereas cyclic isomers are produced below the detection limit

Kinetic data for the reaction of hydroxyl radicals with 1,1,1-trichloroacetaldehyde at 298 ⁺_- 2 K

DEFF Research Database (Denmark)

Barry, J.; Scollard, D.J.; Treacy, J.J.

1994-01-01

technique with OH radical detection both by resonance fluorescence and electron paramagnetic resonance. The results provide a value of k(OH + CCl3CHO) = (1.1 +/- 0.2) x 10(-12) cm3 molecule-1 s-1 at room temperature giving an atmospheric lifetime for CCl3CHO with respect to reaction with OH radicals of 290...

Exo selective enantioselective nitrone cycloadditions.

Science.gov (United States)

Sibi, Mukund P; Ma, Zhihua; Jasperse, Craig P

2004-01-28

We have developed a novel method for accessing exo adducts with high enantioselectivity in nitrone cycloadditions to enoates. Pyrazolidinones proved to be effective achiral templates in the cycloadditions, providing exo adducts typically in >15:1 selectivity and 90-98% ee. The use of Lewis acids that form square planar complexes, such as copper triflate, was important for obtaining high exo selectivity.

Hydroxyl radical reactivity with diethylhydroxylamine

International Nuclear Information System (INIS)

Gorse, R.A. Jr.; Lii, R.R.; Saunders, B.B.

1977-01-01

Diethylhydroxylamine (DEHA) reacts with gas-phase hydroxyl radicals on every third collision, whereas the corresponding reaction in aqueous solution is considerably slower. The high gas-phase reactivity explains the predicted inhibitory effect of DEHA in atmospheric smog processes. Results from the studies in the aqueous phase are helpful in predicting the mechanism of the reaction of DEHA with hydroxyl radicals

Intramolecular transformation of thiyl radicals to α-aminoalkyl radicals: 'ab initio' calculations on homocystein

International Nuclear Information System (INIS)

Chhun, S.; Berges, J.; Bleton, V.; Abedinzadeh, Z.

2000-01-01

One-electron oxidation of thiols by oxidizing radicals leads to the formation of thiyl radical and carbon-centered radicals. It has been shown experimentally that in the absence of oxygen, the thiyl radicals derived from certain thiols of biological interest such as glutathion, cysteine and homocysteine decay rapidly by intramolecular rearrangement reactions into the carbon-centered radical. In the present work we have investigated theoretically the structure and the stability of thiyl and carbon-centered radicals of homocysteine in order to check the possibility of this rearrangement. (author)

Development of a new free radical absorption capacity assay method for antioxidants: aroxyl radical absorption capacity (ARAC).

Science.gov (United States)

Nagaoka, Shin-ichi; Nagai, Kanae; Fujii, Yuko; Ouchi, Aya; Mukai, Kazuo

2013-10-23

A new free radical absorption capacity assay method is proposed with use of an aroxyl radical (2,6-di-tert-butyl-4-(4'-methoxyphenyl)phenoxyl radical) and stopped-flow spectroscopy and is named the aroxyl radical absorption capacity (ARAC) assay method. The free radical absorption capacity (ARAC value) of each tocopherol was determined through measurement of the radical-scavenging rate constant in ethanol. The ARAC value could also be evaluated through measurement of the half-life of the aroxyl radical during the scavenging reaction. For the estimation of the free radical absorption capacity, the aroxyl radical was more suitable than the DPPH radical, galvinoxyl, and p-nitrophenyl nitronyl nitroxide. The ARAC value in tocopherols showed the same tendency as the free radical absorption capacities reported previously, and the tendency was independent of an oxygen radical participating in the scavenging reaction and of a medium surrounding the tocopherol and oxygen radical. The ARAC value can be directly connected to the free radical-scavenging rate constant, and the ARAC method has the advantage of treating a stable and isolable radical (aroxyl radical) in a user-friendly organic solvent (ethanol). The ARAC method was also successfully applied to a palm oil extract. Accordingly, the ARAC method would be useful in free radical absorption capacity assay of antioxidative reagents and foods.

Enantioselective Analysis in instruments onboard ROSETTA/PHILAE and ExoMars

Science.gov (United States)

Hendrik Bredehöft, Jan; Thiemann, Wolfram; Meierhenrich, Uwe; Goesmann, Fred

It has been suggested a number of times in the past, to look for chirality as a biomarker. So far, for lack of appropriate instrumentation, space missions have never included enantioselective analysis. The distinction between enantiomers is of crucial importance to the question of the origin of the very first (pre)biotic molecules. If molecules detected in situ on another celestial body were found to exhibit a chiral bias, this would mean that at least partial asymmetric synthesis could take place abiotically. If this chiral bias should be found to be near 100For the currently flying ESA mission ROSETTA an enantioselective instrument was built, to try for the first time to detect and separate chiral molecules in situ. This instrument is COSAC, the Cometary Sampling and Acquisition Experiment, an enantioselective GCMS device[1,2], which is included in the lander PHLIAE that will eventually in 2014 land on the nucleus of comet 67P/Churyumov-Gerasimenko. A similar but even more powerful type of enantioselective GC-MS is in preparation for ESA's ExoMars mission. This instrument is part of MOMA, the Mars Organic Molecules Analyser. It has the objective of identifying and quantifying chiral organic molecules in surface and subsurface samples of Mars. Currently ExoMars is scheduled for 2018. The newly developed enantioselective technique utilized by both COSAC and MOMA will be described, including sample acquisition, derivatization, and separation in space-resistant chiral stationary capillary columns with time-of-flight mass spectrometric detection. Results of enantioselective analyses of representative test samples with special emphasis on amino acids[3], the building blocks of protein polymers, will be presented and we will discuss potential results of space missions Rosetta and ExoMars. [1] Thiemann W.H.-P., Meierhenrich U.: ESA Mission ROSETTA Will Probe for Chirality of Cometary Amino Acids. Origins of Life and Evolution of Biospheres 31 (2001), 199-210. [2

Reactions of organic free radicals at colloidal silver in aqueous solution. Electron pool effect and water decomposition

International Nuclear Information System (INIS)

Henglein, A.

1979-01-01

Organic free radicals of high negative redox potential such as α-alcohol radicals were found to transfer electrons to colloidal silver particles stabilized by sodium dodecyl sulfate in aqueous solution. The colloidal particles thus became a pool of stored electrons that could reduce water to form hydrogen or react with suitable acceptors in solution. The organic radicals were produced by irradiation, using suitable scavengers for the primary radicals from the radiolysis of the aqueous solvent. The solutions initially contained silver ions at 1 x 10 -4 - 2 x 10 -3 M. At doses below 10 5 rd, the silver ions were completely reduced to form the colloidal catalyst. In this dose range, the corresponding hydrogen yield amounted to 1 molecule per 100 eV. It increased steeply at higher doses up to 3 molecules per 100 eV. The H 2 yield decreased with increasing dose rate and with increasing pH in alkaline solutions. It was highest at a concentration of sodium dodecyl sulfate of 1 x 10 -3 M, i.e., far below the critical micelle concentration of this surfactant. Changes in the absorption spectrum of the colloid are attributed to changes in the size of the silver particles upon charging up with electrons. The competition of radical-colloid reactions with radical-radical deactivation in the bulk of solution or at the surface of the colloidal particles is also discussed. 11 figures

Asymmetric recombination and electron spin relaxation in the semiclassical theory of radical pair reactions

International Nuclear Information System (INIS)

Lewis, Alan M.; Manolopoulos, David E.; Hore, P. J.

2014-01-01

We describe how the semiclassical theory of radical pair recombination reactions recently introduced by two of us [D. E. Manolopoulos and P. J. Hore, J. Chem. Phys. 139, 124106 (2013)] can be generalised to allow for different singlet and triplet recombination rates. This is a non-trivial generalisation because when the recombination rates are different the recombination process is dynamically coupled to the coherent electron spin dynamics of the radical pair. Furthermore, because the recombination operator is a two-electron operator, it is no longer sufficient simply to consider the two electrons as classical vectors: one has to consider the complete set of 16 two-electron spin operators as independent classical variables. The resulting semiclassical theory is first validated by comparison with exact quantum mechanical results for a model radical pair containing 12 nuclear spins. It is then used to shed light on the spin dynamics of a carotenoid-porphyrin-fullerene triad containing considerably more nuclear spins which has recently been used to establish a “proof of principle” for the operation of a chemical compass [K. Maeda, K. B. Henbest, F. Cintolesi, I. Kuprov, C. T. Rodgers, P. A. Liddell, D. Gust, C. R. Timmel, and P. J. Hore, Nature (London) 453, 387 (2008)]. We find in particular that the intriguing biphasic behaviour that has been observed in the effect of an Earth-strength magnetic field on the time-dependent survival probability of the photo-excited C ·+ PF ·− radical pair arises from a delicate balance between its asymmetric recombination and the relaxation of the electron spin in the carotenoid radical

Electron spin resonance study of radicals in irradiated polyethylene

International Nuclear Information System (INIS)

Fujimura, Takashi

1979-02-01

In order to elucidate radiation effect in polyethylene, the nature and behavior of radicals produced in polyethylene and the model compound of polyethylene irradiated at 77 0 K were studied by using electron spin resonance. The structure of radical pairs, which are composed of two radicals produced very closely each other, was investigated in drawn polyethylene and the single crystal of n-eicosane. The radical pairs of intrachain type and interchain type were found in polyethylene and n-eicosane respectively. It was suggested that these two types of radical pairs are the precursors of double bonds and crosslinks respectively. The thermal decay reactions of radicals themselves produced in irradiated polyethylene were investigated. It was made clear that the short range distances between two radicals play an important role in the decay reaction of alkyl radicals at low temperatures. The trapping regions of radicals were studied and it was clarified that allyl radicals, which are produced by the reaction of alkyl radicals with double bonds, are trapped both in the crystalline and non-crystalline regions. (author)

Reaction of misonidazole with DNA radicals and its effect on the template activity of DNA

International Nuclear Information System (INIS)

Endoh, Daiji; Kuwabara, Mikinori; Sato, Fumiaki; Yoshii, Giichi.

1985-01-01

After calf thymus DNA was gamma-irradiated in the solid state in vacuo and subsequently dissolved in aqueous solution containing misonidazole (3 mM) under hypoxic condition, the frequency of single-strand breaks and alkali-labile sites in DNA and the amount of misonidazole bound to DNA were measured. The presence of misonidazole converted the precursor radicals, which otherwise results in single-strand breaks, to alkali-labile sites, and the amount of alkali-labile sites increased linearly with increasing radiation dose. The amount of misonidazole bound to DNA also increased linearly with increasing radiation dose. The biological meaning of the changes in the frequency of single-strand breaks and alkali-labile sites by the reaction of misonidazole with DNA radicals and of binding misonidazole with DNA was examined using a model system to measure the template activity of DNA for RNA synthesis in vitro. The conversion of DNA radicals to alkali-labile sites protected the radiation-induced decrease in the template activity of DNA, while the adduct formation of misonidazole had no effect on it. (author)

Highly Enantioselective Rhodium-Catalyzed Addition of Arylboroxines to Simple Aryl Ketones: Efficient Synthesis of Escitalopram.

Science.gov (United States)

Huang, Linwei; Zhu, Jinbin; Jiao, Guangjun; Wang, Zheng; Yu, Xingxin; Deng, Wei-Ping; Tang, Wenjun

2016-03-24

Highly enantioselective additions of arylboroxines to simple aryl ketones have been achieved for the first time with a Rh/(R,R,R,R)-WingPhos catalyst, thus providing a range of chiral diaryl alkyl carbinols with excellent ee values and yields. (R,R,R,R)-WingPhos has been proven to be crucial for the high reactivity and enantioselectivity. The method has enabled a new, concise, and enantioselective synthesis of the antidepressant drug escitalopram. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Palladium-Catalyzed Enantioselective C-H Olefination of Diaryl Sulfoxides through Parallel Kinetic Resolution and Desymmetrization.

Science.gov (United States)

Zhu, Yu-Chao; Li, Yan; Zhang, Bo-Chao; Zhang, Feng-Xu; Yang, Yi-Nuo; Wang, Xi-Sheng

2018-03-07

The first example of Pd II -catalyzed enantioselective C-H olefination with non-chiral or racemic sulfoxides as directing groups was developed. A variety of chiral diaryl sulfoxides were synthesized with high enantioselectivity (up to 99 %) through both desymmetrization and parallel kinetic resolution (PKR). This is the first report of Pd II -catalyzed enantioselective C(sp 2 )-H functionalization through PKR, and it represents a novel strategy to construct sulfur chiral centers. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Redox properties of free radicals

International Nuclear Information System (INIS)

Neta, P.

1981-01-01

Results of electron transfer reactions observed and monitored by pulse radiolysis are reported. This technique allows determination of the first one-electron reduction or oxidation of a compound rather than the overall two-electron transfer usually reported. Pulse radiolysis allows the determination of absolute rate constants for reactions of free radicals and helps elucidate the mechanisms involved. Studies using this technique to study radicals derived from quinones, nitro compounds, pyridines, phenols, and anilines are reported. Radicals of biochemical interest arising from riboflavin, ascorbic acid, vitamin K 3 , vitamin E, MAD + , porphyrins, etc. have also been studied

Reactions of thiocarbamate, triazine and urea herbicides, RDX and benzenes on EPA Contaminant Candidate List with ozone and with hydroxyl radicals

Science.gov (United States)

Second-order rate constants of the direct ozone reactions (kO3,M) and the indirect OH radical reactions (kOH,M) for nine chemicals on the US EPA’s Drinking Water Contaminant Candidate List (CCL) were studied during the ozonation and ozone/hydrogen peroxide a...

Pulse radiolysis and ab initio SCF MO studies of hydroxyl radical reactions with 2,2'-bipyridine and its complexes with transition metal ions

Energy Technology Data Exchange (ETDEWEB)

Maliyachel, A C

1984-01-01

In the present study, reactions of hydroxyl radical with 2,2'-bipyridine (bpy) and complexes of iron(II) and cobalt(III) containing 2,2'-bipyridine and/or cyanide as ligands have been investigated by pulse radiolysis and also by ab initio self-consistent field, molecular orbital (SCF MO) theoretical techniques for 2,2'-bipyridine and pyridines. In the pulse radiolysis experiments, the nascent products of hydroxyl radical reactions with these compounds have been characterized through their spectral and kinetic properties. All these reactions occur at near diffusion controlled rates to give transient products having absorption in the ultraviolet, visible and, in some cases, near-IR region. The primary reactions of OH are considered to take place by addition mechanisms in the cases of 2,2'-bipyridine, (Fe(bpy)/sub 3/)/sup 2 +/, (Fe(DMbpy)/sub 3/)/sup 2 +/ and (Co(bpy)/sub 3/)/sup 3 +/. With (Fe(pby)/sub 2/(CN)/sub 2/) and (Fe(bpy)(CN)/sub 4/)/sup 2 -/, both addition and charge transfer processes occur. The present study indicates that hydroxyl radical reactions with 2,2'-bipyridine can be considerably altered by complexation with metal ions such as iron(II) and cobalt(III), and the factors associated with this are discussed. In the second part of this work, ab initio SCF MO calculations have been performed for the reactions of OH with pyridine, pyridinium ion and 2,2'-bipyridine. Based on the calculated total energies for the various hydroxy radical products, the relative stability of OH addition products are found to be for pyridine, meta-C > N >> para-C > ortho-C; for pyridinium ion, meta-C >> para-C > ortho-C > N, and for 2,2'- bipyridine, C/sub 5/ > C/sub 6/ > C/sub 3/ > C/sub 4/ > N.

Chemical repair activity of free radical scavenger edaravone: reduction reactions with dGMP hydroxyl radical adducts and suppression of base lesions and AP sites on irradiated plasmid DNA.

Science.gov (United States)

Hata, Kuniki; Urushibara, Ayumi; Yamashita, Shinichi; Lin, Mingzhang; Muroya, Yusa; Shikazono, Naoya; Yokoya, Akinari; Fu, Haiying; Katsumura, Yosuke

2015-01-01

Reactions of edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) with deoxyguanosine monophosphate (dGMP) hydroxyl radical adducts were investigated by pulse radiolysis technique. Edaravone was found to reduce the dGMP hydroxyl radical adducts through electron transfer reactions. The rate constants of the reactions were greater than 4 × 10(8) dm(3) mol(-1) s(-1) and similar to those of the reactions of ascorbic acid, which is a representative antioxidant. Yields of single-strand breaks, base lesions, and abasic sites produced in pUC18 plasmid DNA by gamma ray irradiation in the presence of low concentrations (10-1000 μmol dm(-3)) of edaravone were also quantified, and the chemical repair activity of edaravone was estimated by a method recently developed by the authors. By comparing suppression efficiencies to the induction of each DNA lesion, it was found that base lesions and abasic sites were suppressed by the chemical repair activity of edaravone, although the suppression of single-strand breaks was not very effective. This phenomenon was attributed to the chemical repair activity of edaravone toward base lesions and abasic sites. However, the chemical repair activity of edaravone for base lesions was lower than that of ascorbic acid. © The Author 2014. Published by Oxford University Press on behalf of The Japan Radiation Research Society and Japanese Society for Radiation Oncology.

Cytosine Radical Cations: A Gas-Phase Study Combining IRMPD Spectroscopy, UVPD Spectroscopy, Ion-Molecule Reactions, and Theoretical Calculations

Czech Academy of Sciences Publication Activity Database

Lesslie, M.; Lawler, J. T.; Dang, A.; Korn, J. A.; Bím, Daniel; Steinmetz, V.; Maitre, P.; Tureček, F.; Ryzhov, V.

2017-01-01

Roč. 18, č. 10 (2017), s. 1293-1301 ISSN 1439-4235 Institutional support: RVO:61388963 Keywords : ion-molecule reactions * IRMPD spectroscopy * nucleobases * radical ions * UVPD spectroscopy Subject RIV: CF - Physical ; Theoretical Chemistry OBOR OECD: Physical chemistry Impact factor: 3.075, year: 2016

Muoniated acyl and thioacyl radicals

International Nuclear Information System (INIS)

McKenzie, Iain; Brodovitch, Jean-Claude; Ghandi, Khashayar; Percival, Paul W.

2006-01-01

The product of the reaction of muonium with tert-butylisocyanate was previously assigned as the muoniated tert-butylaminyl radical (I. McKenzie, J.-C. Brodovitch, K. Ghandi, S. Kecman, P. W. Percival, Physica B 326 (2003) 76). This assignment is incorrect since the muon and 14 N hyperfine-coupling constants (hfcc) of this radical would have the opposite sign, which is in conflict with the experimental results. The radical is now reassigned as the muoniated N-tert-butylcarbamoyl radical, based on the similarities between the experimental muon and 14 N hfcc and hfcc calculated at the UB3LYP/6-311G(d,p)//UB3LYP/EPR-III level. The large zero-point energy in the N-Mu bond results in the dissociation barrier of the muoniated N-tert-butylcarbamoyl radical being above the combined energy of the reactants, in contrast to the N-tert-butylcarbamoyl radical where the dissociation barrier lies below the combined energy of the reactants. The reaction of muonium with tert-butylisothiocyanate produced both conformers of the muoniated N-tert-butylthiocarbamoyl radical and their assignment was based on the similarities between the experimental and calculated muon hfcc. These are the first acyl and thioacyl radicals to be directly detected by muon spin spectroscopy

Muoniated acyl and thioacyl radicals

Energy Technology Data Exchange (ETDEWEB)

McKenzie, Iain [TRIUMF and Department of Chemistry, 8888 University Drive, Simon Fraser University, Burnaby B.C., V5A 1S6 (Canada); Brodovitch, Jean-Claude [TRIUMF and Department of Chemistry, 8888 University Drive, Simon Fraser University, Burnaby B.C., V5A 1S6 (Canada); Ghandi, Khashayar [TRIUMF and Department of Chemistry, 8888 University Drive, Simon Fraser University, Burnaby B.C., V5A 1S6 (Canada); Percival, Paul W. [TRIUMF and Department of Chemistry, 8888 University Drive, Simon Fraser University, Burnaby B.C., V5A 1S6 (Canada)]. E-mail: percival@sfu.ca

2006-03-31

The product of the reaction of muonium with tert-butylisocyanate was previously assigned as the muoniated tert-butylaminyl radical (I. McKenzie, J.-C. Brodovitch, K. Ghandi, S. Kecman, P. W. Percival, Physica B 326 (2003) 76). This assignment is incorrect since the muon and {sup 14}N hyperfine-coupling constants (hfcc) of this radical would have the opposite sign, which is in conflict with the experimental results. The radical is now reassigned as the muoniated N-tert-butylcarbamoyl radical, based on the similarities between the experimental muon and {sup 14}N hfcc and hfcc calculated at the UB3LYP/6-311G(d,p)//UB3LYP/EPR-III level. The large zero-point energy in the N-Mu bond results in the dissociation barrier of the muoniated N-tert-butylcarbamoyl radical being above the combined energy of the reactants, in contrast to the N-tert-butylcarbamoyl radical where the dissociation barrier lies below the combined energy of the reactants. The reaction of muonium with tert-butylisothiocyanate produced both conformers of the muoniated N-tert-butylthiocarbamoyl radical and their assignment was based on the similarities between the experimental and calculated muon hfcc. These are the first acyl and thioacyl radicals to be directly detected by muon spin spectroscopy.

The Catalytic Enantioselective Total Synthesis of (+)-Liphagal

KAUST Repository

Day, Joshua J.; McFadden, Ryan M.; Virgil, Scott C.; Kolding, Helene; Alleva, Jennifer L.; Stoltz, Brian M.

2011-01-01

Ring a ding: The meroterpenoid natural product (+)-liphagal has been synthesized enantioselectively in 19 steps from commercially available materials. The trans-homodecalin system was achieved by ring expansion followed by stereoselective hydrogenation. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The Catalytic Enantioselective Total Synthesis of (+)-Liphagal

KAUST Repository

Day, Joshua J.

2011-06-10

Ring a ding: The meroterpenoid natural product (+)-liphagal has been synthesized enantioselectively in 19 steps from commercially available materials. The trans-homodecalin system was achieved by ring expansion followed by stereoselective hydrogenation. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Dynamics of activity free radical oxidation reactions in students with cerebral palsy results over the course of the educational process

Directory of Open Access Journals (Sweden)

Makarova E.V.

2012-12-01

Full Text Available The dynamics of changes activity of reactions is studied freely radical oxidize for students with the consequences of child's cerebral paralysis. 20 students took part in an experiment. Found that the course of study they have more active free radical oxidation reactions and decreases the activity of antiradical protection. Given the use of additional physical activity in aerobic training indicators intracellular antioxidant defense system increased, decreased content of reaction products of lipid peroxidation. However, increased rates of maximum oxygen consumption and increased tolerance of students with cerebral palsy to the consequences of physical activity. It is set that the pathological changes of metabolism for students ground the necessity of application of the differentiated physical loadings. The optimum forms of physical rehabilitation of the aerobic training is the dosed walking, medical swimming, dosed after distance, sometimes and by the corner of getting up pedestrian ascents. Loading is increased due to a volume, but not intensity of exercises.

Using chiral ionic liquid additives to enhance asymmetric induction in a Diels-Alder reaction.

Science.gov (United States)

Goodrich, P; Nimal Gunaratne, H Q; Hall, L; Wang, Y; Jin, L; Muldoon, M J; Ribeiro, A P C; Pombeiro, A J L; Pârvulescu, V I; Davey, P; Hardacre, C

2017-01-31

A bis-oxazoline ligand has been complexed using Cu(ii) and Zn(ii) trifluoromethanesulfonate and a range of chiral ionic liquid (CIL) additives based on natural products were used as a co-catalyst for a Diels-Alder reaction. The catalytic performance of these systems was compared for the asymmetric Diels-Alder reaction between N-acryloyloxazolidinone and cyclopentadiene with and without the presence of a CIL additive. In the absence of the CIL, both catalysts resulted in low enantioselectivities in conventional solvents and ionic liquids. However, whilst only a minor effect of the CIL was observed for the Cu based catalyst, in the case of the Zn based catalyst, significant enhancements in endo enantioselectivity of up to 50% were found on the addition of a CIL.

Enantioselective conjugate addition of diethylzinc to chalcone catalyzed by Co(acac)2 and chiral amino alcohols

NARCIS (Netherlands)

Vries, André H.M. de; Feringa, Bernard

1997-01-01

Co(acac)2 in the presence of chiral ligands has been employed as catalyst for the enantioselective conjugate addition of diethylzinc to chalcone. With chiral amino alcohols derived from (+)-camphor, enantioselectivities up to 83% were achieved.

Kinetics of free radical decay reactions in lactic acid homo and copolymers irradiated to sterilization dose

International Nuclear Information System (INIS)

Kantoglu, O.; Ozbey, T.; Gueven, O.

1995-01-01

The kinetics of free radical decay reactions of poly(L-Lactic acid), poly(DL-Lactic acid) and random copolymer of lactic and glycolic acid have been investigated for decays taking place in air and in vacuum. The change in ESR lines of γ-irradiated polymers have been followed over a long time period. The decay has been found to follow neither simple first-order nor second-order kinetics. Various kinetic approaches including composite first or second-order mechanisms and diffusion-controlled first or second-order equations were determined to be also unsatisfactory. The decay of radicals in bulk irradiated lactic acid homo and copolymers was found to be best described when the second-order non-classical equation with time dependent rate constant approach was used. (Author)

THE HYDROVINYLATION AND RELATED REACTIONS: NEW PROTOCOLS AND CONTROL ELEMENTS IN SEARCH OF GREATER SYNTHETIC EFFICIENCY AND SELECTIVITY. (R826120)

Science.gov (United States)

New reaction conditions and stereochemical control elements for heterodimerization between ethylene (or propylene) and functionalized vinyl arenes are highlighted (see equation). For example, an enantioselective version of the hydrovinylation reaction uses [{(allyl)NiBr}...

Sulfate radical-based water treatment in presence of chloride: formation of chlorate, inter-conversion of sulfate radicals into hydroxyl radicals and influence of bicarbonate.

Science.gov (United States)

Lutze, Holger V; Kerlin, Nils; Schmidt, Torsten C

2015-04-01

Sulfate radical (SO4(-)) based oxidation is discussed as a potential water treatment option and is already used in ground water remediation. However, the complex SO4(-) chemistry in various matrices is poorly understood. In that regard, the fast reaction of SO4(-) with Cl(-) is of high importance since Cl(-) belongs to the main constituents in aqueous environments. This reaction yields chlorine atoms (Cl) as primary products. Cl initiate a cascade of subsequent reactions with a pH dependent product pattern. At low pH ( 5 Cl mainly react with water yielding hydroxyl radicals. Thus, at moderate Cl(-) concentrations (mM range) the SO4(-)-based process may be converted into a conventional (hydroxyl radical -based) advanced oxidation process. The conversion of SO4(-) into OH, however, is interrupted in presence of bicarbonate by scavenging of Cl. Copyright © 2014. Published by Elsevier Ltd.

Crotylations of alpha-carbonyl radicals with crotylstannane.

Science.gov (United States)

Sibi, Mukund P; Miyabe, Hideto

2002-10-03

Electrophilic radicals undergo crotylation with crotylstannane with moderate to good efficiency. The reaction provides the syn isomer as the major product. The present methodology is complementary to Claisen protocols for the synthesis of gamma,delta-unsaturated carboxylic acid derivatives. Details of the new radical methodology are presented. [reaction: see text

Self-Terminating, Oxidative Radical Cyclizations

Directory of Open Access Journals (Sweden)

Uta Wille

2004-05-01

Full Text Available The recently discovered novel concept of self-terminating, oxidative radical cyclizations, through which alkynes can be converted into carbonyl compounds under very mild reaction conditions using O-centered inorganic and organic radicals as oxidants, is described

Reactions of clofibric acid with oxidative and reductive radicals-Products, mechanisms, efficiency and toxic effects

Science.gov (United States)

Csay, Tamás; Rácz, Gergely; Salik, Ádám; Takács, Erzsébet; Wojnárovits, László

2014-09-01

The degradation of clofibric acid induced by hydroxyl radical, hydrated electron and O2-•/HO2• reactive species was studied in aqueous solutions. Clofibric acid was decomposed more effectively by hydroxyl radical than by hydrated electron or O2-•/HO2•. Various hydroxylated, dechlorinated and fragmentation products have been identified and quantified. A new LC-MS method was developed based on 18O isotope labeling to follow the formation of hydroxylated derivatives of clofibric acid. Possible degradation pathways have been proposed. The overall degradation was monitored by determination of sum parameters like COD, TOC and AOX. It was found that the organic chlorine degrades very effectively prior to complete mineralization. After the treatment no toxic effect was found according to Vibrio fischeri tests. However, at early stages some of the reaction products were more harmful than clofibric acid.

Guanidine-catalyzed enantioselective desymmetrization of meso-aziridines

KAUST Repository

Zhang, Yan

2011-01-01

An amino-indanol derived chiral guanidine was developed as an efficient Brønsted base catalyst for the desymmetrization of meso-aziridines with both thiols and carbamodithioic acids as nucleophiles, which provided 1,2-difunctionalized ring-opened products in high yields and enantioselectivities. © The Royal Society of Chemistry.

Enantioselective Epoxide Polymerization Using a Bimetallic Cobalt Catalyst

KAUST Repository

Thomas, Renee M.

2010-11-24

A highly active enantiopure bimetallic cobalt complex was explored for the enantioselective polymerization of a variety of monosubstituted epoxides. The polymerizations were optimized for high rates and stereoselectivity, with s-factors (kfast/kslow) for most epoxides exceeding 50 and some exceeding 300, well above the threshold for preparative utility of enantiopure epoxides and isotactic polyethers. Values for mm triads of the resulting polymers are typically greater than 95%, with some even surpassing 98%. In addition, the use of a racemic catalyst allowed the preparation of isotactic polyethers in quantitative yields. The thermal properties of these isotactic polyethers are presented, with many polymers exhibiting high T m values. This is the first report of the rapid synthesis of a broad range of highly isotactic polyethers via the enantioselective polymerization of racemic epoxides. © 2010 American Chemical Society.

Enantioselective Epoxide Polymerization Using a Bimetallic Cobalt Catalyst

KAUST Repository

Thomas, Renee M.; Widger, Peter C. B.; Ahmed, Syud M.; Jeske, Ryan C.; Hirahata, Wataru; Lobkovsky, Emil B.; Coates, Geoffrey W.

2010-01-01

A highly active enantiopure bimetallic cobalt complex was explored for the enantioselective polymerization of a variety of monosubstituted epoxides. The polymerizations were optimized for high rates and stereoselectivity, with s-factors (kfast/kslow) for most epoxides exceeding 50 and some exceeding 300, well above the threshold for preparative utility of enantiopure epoxides and isotactic polyethers. Values for mm triads of the resulting polymers are typically greater than 95%, with some even surpassing 98%. In addition, the use of a racemic catalyst allowed the preparation of isotactic polyethers in quantitative yields. The thermal properties of these isotactic polyethers are presented, with many polymers exhibiting high T m values. This is the first report of the rapid synthesis of a broad range of highly isotactic polyethers via the enantioselective polymerization of racemic epoxides. © 2010 American Chemical Society.

Enantioselective conjugate addition of diethylzinc to chalcone catalyzed by Co(acac)(2) and chiral amino alcohols

NARCIS (Netherlands)

de Vries, A.H.M.; Feringa, B.L.

1997-01-01

Co(acac)(2) in the presence of chiral ligands has been employed as catalyst for the enantioselective conjugate addition of diethylzinc to chalcone. With chiral amino alcohols derived from (+)-camphor, enantioselectivities up to 83% were achieved. (C) 1997 Elsevier Science Ltd.

Influence of biochar on the enantioselective behavior of the chiral fungicide metalaxyl in soil

Science.gov (United States)

Gámiz, Beatriz; Pignatello, Joseph J.; Hermosín, María Carmen; Cox, Lucía; Celis, Rafael

2015-04-01

Chiral pesticides comprise an emerging and important class of organic pollutants currently, accounting for more than a quarter of used pesticides. Consequently, the contamination problems caused by chiral pesticides are concern matter and factors affecting enantioselective processes of chiral pesticides in soil need to be understood. For example, certain soil management practices, such as the use of organic amendments, can affect the enantioselective behavior of chiral pesticides in soils. Recently, biochar (BC), i.e. organic matter subjected to pyrolysis, has been proposed as organic amendment due to beneficial properties such as its high stability against decay in soil environments and its apparent ability to influence the availability of nutrients. BC is considered to be more biologically inert as compared to otherforms of organic carbon. However, its side-effects on the enantioselectivity of processes affecting the fate of chiral pesticides is unknown. The aim of this study was to assess the effect of biochar (BC) on the enantioselectivity of sorption, degradation, and leaching of the chiral fungicide metalaxyl in an agricultural soil. Amending the soil with BC (2% w/w) resulted in 3 times higher sorption of metalaxyl enantiomers compared to unamended soil, but no enantioselectivity in the process was observed. Moreover, both enantiomers showed some resistance to be desorbed in BC-amended soil compared to unamended soil. Dissipation studies revealed that the degradation of metalaxylwas more enantioselective in the unamended soil than in BC-amended soil. In unamended soil, R-metalaxyl(biologically active) and S- metalaxyl had half-lives (t1/2) of 3 and 34 days, respectively. BC enhanced the persistence of both enantiomers in the soil, with R-metalaxyl being degraded faster (t1/2=43 days) than S-metalaxyl (t1/2= 100 days). The leaching of both S-and R-metalaxyl was almost suppressed after amending the soil with BC; less than 10% of the fungicide applied to soil

Kinetics of free radical reactions in irradiated crystalline L-leucine

International Nuclear Information System (INIS)

Dole, M.; Mahdavi, M.

1983-01-01

Four aspects of the radiation chemistry of crystalline L-leucine are presented. They are as follows: the transformation of one type of free radical into another as observed by following the ESR spectrum of individual peaks in the initial stages of the free radical decay at room or higher temperatures after a gamma irradiation at 77K; the catalytic effect of hydrogen gas in significantly accelerating the free radical decay; the unexpected effect of argon gas in decreasing the rate of the free radical decay; and the accurate agreement of the decay data with the diffusion controlled second order Waite equation when the latter is expressed in the linear form. The major free radical which exists in irradiated L-leucine gives rise to a spectrum of 8 Doublets for a total of 16 lines. Numbering the peaks from the least intense to the strongest, peaks numbers 3, 4, 6, and 7, initially increase in intensity before decreasing while peak number 5 rapidly decreases. However, during this initial period the total free radical concentration as measured by the total moment of the ESR spectrum decreases. The kinetics of these effects are discussed. (author)

Kinetics of the Reactions of IO Radicals with NO and NO2

Science.gov (United States)

Daykin, E. P.; Wine, P. H.

1997-01-01

A laser flash photolysis-long path absorption technique has been employed to study the kinetics of the reactions of IO radicals with NO and NO2 as a function of temperature and pressure. The IO and NO rate coefficient is independent of pressure over the range 40-200 Torr of N2, and its temperature dependence over the range 242-359 K is adequately described by the Arrhenius expression k(sub 1) = (6.9 +/- 1.7) x 10(exp -12) exp[(328 +/- 71)/T] cu cm/(molecule.s) (errors are 2 sigma, precision only). These Arrhenius parameters are similar to those determined previously for the ClO + NO and BrO + NO reactions. The IO and NO2 association reaction is found to be in the falloff regime over the temperature and pressure ranges investigated (254-354 K and 40-750 Torr of N2). Assuming F(sub c) = 0.4 independent of temperature, a physically reasonable set of falloff parameters which adequately describe the data are k(sub 0) = 7.7 x 10(exp -31)(T/300)(exp -5.0) cm(exp 6)/(molecule(exp 2).s) and k(sub infinity) = 1.55 x 10(exp -11)cu cm/(molecule.s) independent of temperature. The IO + NO2 rate coefficients determined in this study are about a factor of 2 faster than those reported in the only previous study of this reaction.

A barrier-free atomic radical-molecule reaction: N (2D) NO2 (2A1) mechanistic study

Science.gov (United States)

Zuo, Ming-Hui; Liu, Hui-Ling; Huang, Xu-Ri; Zhan, Jin-Hui; Sun, Chia-Chung

The reaction of N (2D) radical with NO2 molecule has been studied theoretically using density functional theory and ab initio quantum chemistry method. Singlet electronic state [N2O2] potential energy surfaces (PES) are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311+G(d) + ZPE and G3B3 levels of theory. All the involved transition states for generation of (2NO) and (O2 + N2) lie much lower than the reactants. Thus, the novel reaction N + NO2 can proceed effectively even at low temperatures and it is expected to play a role in both combustion and interstellar processes. On the basis of the analysis of the kinetics of all pathways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the title reaction.

Radical-pair based avian magnetoreception

Science.gov (United States)

Procopio, Maria; Ritz, Thorsten

2014-03-01

Behavioural experiments suggest that migratory birds possess a magnetic compass sensor able to detect the direction of the geomagnetic. One hypothesis for the basis of this remarkable sensory ability is that the coherent quantum spin dynamics of photoinduced radical pair reactions transduces directional magnetic information from the geomagnetic field into changes of reaction yields, possibly involving the photoreceptor cryptochrome in the birds retina. The suggested radical-pair based avian magnetoreception has attracted attention in the field of quantum biology as an example of a biological sensor which might exploit quantum coherences for its biological function. Investigations on such a spin-based sensor have focussed on uncovering the design features for the design of a biomimetic magnetic field sensor. We study the effects of slow fluctuations in the nuclear spin environment on the directional signal. We quantitatively evaluate the robustness of signals under fluctuations on a timescale longer than the lifetime of a radical pair, utilizing two models of radical pairs. Our results suggest design principles for building a radical-pair based compass sensor that is both robust and highly directional sensitive.

Evaluation of reversible interconversion in comprehensive two-dimensional gas chromatography using enantioselective columns in first and second dimensions.

Science.gov (United States)

Kröger, Sabrina; Wong, Yong Foo; Chin, Sung-Tong; Grant, Jacob; Lupton, David; Marriott, Philip J

2015-07-24

The reversible molecular interconversion behaviour of a synthesised oxime (2-phenylpropanaldehyde oxime; (C6H5)CH(CH3)CHN(OH)) was investigated by both, single dimensional gas chromatography (1D GC) and comprehensive two-dimensional gas chromatography (GC×GC). Previous studies on small molecular weight oximes were extended to this larger aromatic oxime (molar mass 149.19gmol(-1)) with interest in the extent of interconversion, enantioselective resolution, and retention time. On a polyethylene glycol (PEG; wax-type) column, a characteristic interconversion zone between two antipodes of E and Z isomers was formed by molecules which have undergone isomerisation on the column (E⇌Z). The extent of interconversion was investigated by varying chromatographic conditions (oven temperature and carrier flow rate) to understand the nature of the behaviour observed. The extent of interconversion was negligible in both enantioselective and methyl-phenylpolysiloxane phase-columns, correlating with the low polarity of the stationary phase. In order to obtain isomerisation along with enantio-resolution, a wax-type and an enantioselective column were coupled in either enantioselective-wax or wax-enantioselective order. The most appropriate column arrangement was selected for study by using a GC×GC experiment with either a wax-phase or phenyl-methylpolysiloxane phase as (2)D column. In addition to evaluation of these fast elution columns, a long narrow-bore enantioselective column (10m) was introduced as (2)D, providing an enantioselective-PEG (coupled-column ensemble: (1)D1+(1)D2)×enantioselective ((2)D) column combination. In this instance, the (1)D1 enantioselective column provides enantiomeric separation of the corresponding enantiomers ((R) and (S)) of (E)- and (Z)-2-phenylpropanaldehyde oxime, followed by E/Z isomerisation in the coupled (1)D2 PEG (reactor) column. The resulting chromatographic interconversion region was modulated and separated into either E/Z isomers

Hexafluorobenzene: A powerful solvent for a noncovalent stereoselective organocatalytic Michael addition reaction

KAUST Repository

Lattanzi, Alessandra

2012-01-01

A dramatic enhancement of the diastereo- and enantioselectivity in the nitro-Michael addition reaction organocatalysed by a commercially available α,α-l-diaryl prolinol was disclosed when performing the reaction in unconventional hexafluorobenzene as a medium. DFT calculations were performed to clarify the origin of stereoselectivity and the role of C 6F 6. © The Royal Society of Chemistry 2012.

Cation radicals of xanthophylls.

Science.gov (United States)

Galinato, Mary Grace I; Niedzwiedzki, Dariusz; Deal, Cailin; Birge, Robert R; Frank, Harry A

2007-10-01

Carotenes and xanthophylls are well known to act as electron donors in redox processes. This ability is thought to be associated with the inhibition of oxidative reactions in reaction centers and light-harvesting pigment-protein complexes of photosystem II (PSII). In this work, cation radicals of neoxanthin, violaxanthin, lutein, zeaxanthin, beta-cryptoxanthin, beta-carotene, and lycopene were generated in solution using ferric chloride as an oxidant and then studied by absorption spectroscopy. The investigation provides a view toward understanding the molecular features that determine the spectral properties of cation radicals of carotenoids. The absorption spectral data reveal a shift to longer wavelength with increasing pi-chain length. However, zeaxanthin and beta-cryptoxanthin exhibit cation radical spectra blue-shifted compared to that of beta-carotene, despite all of these molecules having 11 conjugated carbon-carbon double bonds. CIS molecular orbital theory quantum computations interpret this effect as due to the hydroxyl groups in the terminal rings selectively stabilizing the highest occupied molecular orbitals of preferentially populated s-trans-isomers. The data are expected to be useful in the analysis of spectral results from PSII pigment-protein complexes seeking to understand the role of carotene and xanthophyll cation radicals in regulating excited state energy flow, in protecting PSII reaction centers against photoinhibition, and in dissipating excess light energy absorbed by photosynthetic organisms but not used for photosynthesis.

Allylthioketone Mediated Free Radical Polymerization of Methacrylates

Directory of Open Access Journals (Sweden)

Feng Zhong

2017-11-01

Full Text Available By combination of high trapping free radical efficiency of the thioketone and resonance of the allylic radical, a new type of mediating agent, 1,3,3-triphenylprop-2-ene-1-thione (TPPT has been successfully synthesized, and then is used to study controlled free radical polymerization of methacrylates. Very stable TPPT radicals at the end of poly(methyl methacrylate (PMMA are detected in the polymerization of MMA using TPPT and AIBN as the control agent and initiator. The MALDI-TOF MS spectra are used to identify terminal groups of the resultant poly(glycidyl methacrylate (PGMA, and major component of the obtained polymer has the structure, (CH32(CNC-PGMA-C7H9O3. Chain extension reaction tests ascertain formation of the dead polymers during the polymer storage and purification process of the polymers. Owing to very slow fragmentation reaction of the TPPT-terminated polymethacrylate radical and addition reaction of this radical with a primary radical, the growing chain radicals are difficult to be regenerated, leading to an unobvious change of the molecular weight with monomer conversion. The molecular weights of polymers can be controlled by the ratios of monomer/initiator and TPPT/initiator. However, the first order kinetics of the polymerization and the polymers with narrow polydispersity are obtained, and these phenomena are discussed. This study provides useful information on how to design a better controlling agent.

Rate Constants and Activation Energies for Gas-Phase Reactions of Three Cyclic Volatile Methyl Siloxanes with the Hydroxyl Radical.

Science.gov (United States)

Safron, Andreas; Strandell, Michael; Kierkegaard, Amelie; Macleod, Matthew

2015-07-01

Reaction with hydroxyl radicals (OH) is the major pathway for removal of cyclic volatile methyl siloxanes (cVMS) from air. We present new measurements of second-order rate constants for reactions of the cVMS octamethylcyclotetrasiloxane (D 4 ), decamethylcyclopentasiloxane (D 5 ), and dodecamethylcyclohexasiloxane (D 6 ) with OH determined at temperatures between 313 and 353 K. Our measurements were made using the method of relative rates with cyclohexane as a reference substance and were conducted in a 140-mL gas-phase reaction chamber with online mass spectrometry analysis. When extrapolated to 298 K, our measured reaction rate constants of D 4 and D 5 with the OH radical are 1.9 × 10 -12 (95% confidence interval (CI): (1.7-2.2) × 10 -12 ) and 2.6 × 10 -12 (CI: (2.3-2.9) × 10 -12 ) cm 3 molecule -1 s -1 , respectively, which are 1.9× and 1.7× faster than previous measurements. Our measured rate constant for D 6 is 2.8 × 10 -12 (CI: (2.5-3.2) × 10 -12 ) cm 3 molecule -1 s -1 and to our knowledge there are no comparable laboratory measurements in the literature. Reaction rates for D 5 were 33% higher than for D 4 (CI: 30-37%), whereas the rates for D 6 were only 8% higher than for D 5 (CI: 5-10%). The activation energies of the reactions of D 4 , D 5 , and D 6 with OH were not statistically different and had a value of 4300 ± 2800 J/mol.

Mechanisms of free radical-induced damage to DNA.

Science.gov (United States)

Dizdaroglu, Miral; Jaruga, Pawel

2012-04-01

Endogenous and exogenous sources cause free radical-induced DNA damage in living organisms by a variety of mechanisms. The highly reactive hydroxyl radical reacts with the heterocyclic DNA bases and the sugar moiety near or at diffusion-controlled rates. Hydrated electron and H atom also add to the heterocyclic bases. These reactions lead to adduct radicals, further reactions of which yield numerous products. These include DNA base and sugar products, single- and double-strand breaks, 8,5'-cyclopurine-2'-deoxynucleosides, tandem lesions, clustered sites and DNA-protein cross-links. Reaction conditions and the presence or absence of oxygen profoundly affect the types and yields of the products. There is mounting evidence for an important role of free radical-induced DNA damage in the etiology of numerous diseases including cancer. Further understanding of mechanisms of free radical-induced DNA damage, and cellular repair and biological consequences of DNA damage products will be of outmost importance for disease prevention and treatment.

Enantioselective properties of induced lipases from Geotrichum

Czech Academy of Sciences Publication Activity Database

Zarevúcka, Marie; Kejík, Z.; Šaman, David; Wimmer, Zdeněk; Demnerová, K.

2005-01-01

Roč. 37, - (2005), s. 481-486 ISSN 0141-0229 R&D Projects: GA MŠk(CZ) OC D30.001; GA MŠk(CZ) OC D13.10 Institutional research plan: CEZ:AV0Z40550506 Keywords : Geotrichum * lipase * enantioselectivity Subject RIV: CC - Organic Chemistry Impact factor: 1.705, year: 2005

Gamma-induced reactions of bromo-ethane with olefines. Addition of ethyl radicals to hexene-1 and propylene

International Nuclear Information System (INIS)

Myshkin, V.E.; Shostenko, A.G.; Zagorets, P.A.; Pchelkin, A.I.; Markova, K.G.

1978-01-01

Radiation interaction of bromo-ethane with propylene and 1-hexene has been studied with the aim to investigate the action of γ-radiation on bromalkanes. The absorbed dose rate is 50 rad/s. The reaction products separated by preparative chromatography have been identified with infrared spectroscopy, elemental, chromatographic, and other physico-chemical methods of analysis. It has been established that the reaction with propylene gives rise to telomers whereas interaction of bromo-ethane with 1-hexene yields only the addition product (4-bromoctane). The activation energy of the reactions of adding ethyl radicals to 1-hexene and propylene has been found equal to (3.8+-0.4 kcal/mol) and (2.2+-0.2 kcal/mol), respectively. The activation energy of the reaction of chain transfer through bromo-ethane is (3.7+-0.3 kcal/mol.)

Direct detection of OH formation in the reactions of HO2 with CH3C(OO2 and other substituted peroxy radicals

Directory of Open Access Journals (Sweden)

J. N. Crowley

2008-08-01

Full Text Available This work details the first direct observation of OH as a product from (R1: HO2+CH3C(OO2→(products, which has generally been considered an atmospheric radical termination process. The technique of pulsed laser photolysis radical generation, coupled to calibrated laser induced fluorescence detection was used to measure an OH product yield for (R1 of α1(298 K=(0.5±0.2. This study of (R1 included the measurement of a rate coefficient k1(298 K=(1.4±0.5×10−11cm3 molecule−1 s−1, substantially reducing the uncertainties in modelling this important atmospheric reaction. OH was also detected as a product from the reactions of HO2 with three other carbonyl-containing peroxy radicals, albeit at smaller yield, e.g. (R2: HO2+CH3C(OCH2O2→(products, α2≈0.15. By contrast, OH was not observed (α<0.06 as a major product from reactions where carbonyl functionality was absent, e.g. HO2+HOCH2CH2O2 (R8, and HO2+CH3CH(OHCH2O2 (R9.

Kinetics and mechanisms of the reactions of alkyl radicals with oxygen and with complexes of Co(III), Ru(III), and Ni(III)

International Nuclear Information System (INIS)

Kelley, D.

1990-01-01

The kinetics of the reactions of C 2 H 5 radical with Co(NH 3 ) 5 X 2+ , Ru(NH 3 ) 5 X 2+ , and Co(dmgH) 2 (X) (Y) (X = Br, Cl, N 3 , SCN; Y = H 2 O, CH 3 CN) complexes were studied using laser flash photolysis of ethylcobalt complexes. The kinetics were obtained by the kinetic probe method. Some relative rate constants were also determined by a competition method based on ethyl halide product ratios. The kinetics of colligation reactions of a series of alkyl radicals with β-Ni(cyclam) 2+ were studied using flaser flash photolysis of alkylcobalt complexes. Again, the kinetics were obtained by employing the kinetic probe competition method. The kinetics of the unimolecular homolysis of a series of RNi(cyclam)H 2 O 2+ were studied. Activation parameters were obtained for the unimolecular homolysis of C 2 H 5 Ni(cyclam)H 2 O 2+ . Kinetic and thermodynamic data obtained from these reactions were compared with those for the σ-bonded organometallic complexes. The kinetics of the unimolecular homolysis of a series of RNi(cyclam)H 2 O 2+ complexes were studied by monitoring the formation of the oxygen insertion product RO 2 Ni(cyclam)H 2 O 2+ . The higher rate constants for the reactions of alkyl radicals with oxygen in solution, as compared with those measured in the gas phase, were discussed. 30 refs

Formation of Nitriles in the Interstellar Medium via Reactions of Cyano Radicals, CN(X2Σ+), with Unsaturated Hydrocarbons

Science.gov (United States)

Balucani, N.; Asvany, O.; Huang, L. C. L.; Lee, Y. T.; Kaiser, R. I.; Osamura, Y.; Bettinger, H. F.

2000-12-01

Crossed molecular beam experiments of cyano radicals, CN(X2Σ+, ν=0), in their electronic and vibrational ground state reacting with unsaturated hydrocarbons acetylene, C2H2(X1Σ+g), ethylene, C2H4(X1Ag), methylacetylene, CH3CCH(X1A1), allene, H2CCCH2(X1A1), dimethylacetylene, CH3CCCH3(X1A1'), and benzene, C6H6 (X1A1g), were performed at relative collision energies between 13.3 and 36.4 kJ mol-1 to unravel the formation of unsaturated nitriles in the outflows of late-type AGB carbon stars and molecular clouds. In all reactions, the CN radical was found to attack the π electron density of the hydrocarbon molecule with the radical center located at the carbon atom; the formation of an initial addition complex is a prevalent pathway on all the involved potential energy surfaces. A subsequent carbon-hydrogen bond rupture yields the nitriles cyanoacetylene, HCCCN (X1Σ+), vinylcyanide, C2H3CN (X1A'), 1-methylcyanoacetylene, CH3CCCN (X1A1), cyanoallene, H2CCCH(CN) (X1A'), 3-methylcyanoacetylene, HCCCH2CN(X1A'), 1,1-cyanomethylallene, H2CCC(CN)(CH3) (X1A'), and cyanobenzene, C6H5CN (X1A1). In case of acetylene and ethylene, a second reaction channel involves a [1, 2]-H atom shift in the initial HCCHCN and H2CCH2CN collision complexes prior to a hydrogen atom release to form cyanoacetylene, HCCCN (X1Σ+), and vinylcyanide, C2H3CN (X1A'). Since all these radical-neutral reactions show no entrance barriers, have exit barriers well below the energy of the reactant molecules, and are exothermic, the explicit identification of this CN versus H atom exchange pathway under single collision conditions makes this reaction class a compelling candidate to synthesize unsaturated nitriles in interstellar environments holding temperatures as low as 10 K. This general concept makes it even feasible to predict the formation of nitriles once the corresponding unsaturated hydrocarbons are identified in the interstellar medium. Here HCCCN, C2H3CN, and CH3CCCN have been already observed

Radical cations of quadricyclane and norbornadiene in polar ZSM-5 matrices: Radical cation photochemical transformations without photons

International Nuclear Information System (INIS)

Barnabas, M.V.; Trifunac, A.D.

1994-01-01

Radical cations of quadricyclane (Q) and norbornadiene (NBD) are produced by γ-radiolysis in zeolites. In polar ZSM-5, only one radical cation is initially observed below 100K. Increasing the temperature above 200K gives rise to the cyclopentadiene radical cation. Higher temperatures (>360K) give rise to the cyclopenten-4-yl radical. The observation of cyclopentadiene radical cation implies the occurrence of the reverse Diels-Alder reaction. This is a thermally forbidden, photochemically allowed, process, which is made possible by the interaction of the polar zeolite matrix sites with parent NBD and Q radical cations

A New Mn–Salen Micellar Nanoreactor for Enantioselective Epoxidation of Alkenes in Water

Directory of Open Access Journals (Sweden)

Francesco P. Ballistreri

2018-03-01

Full Text Available A new chiral Mn–salen catalyst, functionalized with a long aliphatic chain and a choline group, able to act as surfactant catalyst for green epoxidation in water, is here described. This catalyst was employed with a commercial surfactant (CTABr leading to a nanoreactor for the enantioselective epoxidation of some selected alkenes in water, using NaClO as oxidant. This is the first example of a nanoreactor for enantioselective epoxidation of non-functionalized alkenes in water.

13C, 18O, and D Fractionation Effects in the Reactions of CH3OH Isotopologues with Cl andOH Radicals

DEFF Research Database (Denmark)

Feilberg, Karen; Gruber-Stadler, Margaret; Johnson, Matthew Stanley

2008-01-01

A relative rate experiment is carried out for six isotopologues of methanol and their reactions with OH and Cl radicals. The reaction rates of CH2DOH, CHD2OH, CD3OH, 13CH3OH, and CH3 18OH with Cl and OH radicals are measured by long-path FTIR spectroscopy relative to CH3OH at 298 ( 2 K and 1013...... with measured high-resolution infrared spectra as references. The relative reaction rates defined as R ) klight/kheavy are determined to be: kOH + CH3OH/kOH + 13CH3OH ) 1.031 ( 0.020, kOH + CH3OH/kOH + CH3 18OH ) 1.017 ( 0.012, kOH + CH3OH/kOH + CH2DOH ) 1.119 ( 0.045, kOH + CH3OH/kOH + CHD2OH ) 1.326 ( 0...... reactions with CH3OH were further investigated using canonical variational transition state theory with small curvature tunneling and compared to experimental measurements as well as to those observed in CH4 and several other substituted methane species. Udgivelsesdato: 16 August 2008...

Theoretical study on the mechanism and kinetics of the reaction of 2,2‧,4,4‧-tetrabrominated diphenyl ether (BDE-47) with OH radicals

Science.gov (United States)

Cao, Haijie; He, Maoxia; Han, Dandan; Sun, Yanhui; Xie, Ju

2011-03-01

The mechanism and kinetic properties of OH-initiated gas-phase reaction of 2,2',4,4'-tetrabrominated diphenyl ether (BDE-47) have been studied at the MPWB1K/6-311 + G(3df,2p)//MPWB1K/6-31G(d) level of theory. Two types of reactions including hydroxyl addition and hydrogen abstraction have been considered. The calculation results indicate that addition reactions except for the bromo-substituted addition reaction have lower energy barriers than hydrogen abstraction reactions. Hydroxyl radical is most likely to be added to non-substituted C atoms (C(3), C(5), and C(6) atoms). Moreover, for all the reactions with OH radical, the bromo-substituted C atoms encountered the highest barrier compared to that of the rest C atoms. The rate constants and product branching ratios of each pathway have been deduced over a wide range of 200-1000 K using canonical variational transition state (CVT) theory with small curvature tunneling (SCT) contribution. This study can be regarded as an attempt to investigate the OH-initiated photochemical reaction mechanism of polybrominated diphenyl ethers (PBDEs).

Reaction pathway of the degradation of the p-hydroxybenzoic acid by sulfate radical generated by ionizing radiations

International Nuclear Information System (INIS)

Criquet, Justine; Leitner, Nathalie Karpel Vel

2015-01-01

The degradation of p-hydroxybenzoic acid (HBA) in aqueous solutions by ionizing radiation was studied. The phenolic pollutant was easily removed by the electron beam irradiation, as more than 80% of the initial 100 µM introduced was degraded for a dose of 600 Gy. It was shown that the addition of persulfate, producing the sulfate radical as additional reactive species, induced a change in the reaction pathway. LC–MS analyses were performed in order to identify the different by-products formed. In the absence of persulfate, the main by-product formed was 3,4-dihydroxybenzoic acid, while in presence of persulfate, 1,4-benzoquinone was detected and the hydroxylated by-products were not present. A reaction pathway of HBA degradation by hydroxyl and sulfate radicals was proposed from the identification of the chemical structure of the different by-products detected. The influences of pH and dissolved oxygen were also studied. A high decline of HBA degradation was observed at pH 11 compared to pH 4.5, this decrease was minimized in the presence of persulfate. The dissolved oxygen concentration was found to be a limiting parameter of HBA degradation, however an excess of dissolved oxygen in solution did not improve the degradation to a large extent. - Highlights: • p-Hydroxybenzoic acid (HBA) is easily removed by e-beam irradiation. • The sulfate radicals formed from persulfate induce loss of the benzoic acid skeleton. • The dissolved oxygen concentration is a limiting parameter of the HBA degradation. • The effect of pH is minimized in presence of persulfate

Intramolecular and Transannular Diels-Alder Reactions

DEFF Research Database (Denmark)

Tanner, David Ackland; Ascic, Erhad

2014-01-01

Few reactions can compete with the Diels-Alder (DA) [4+2] cycloaddition for the rapid and efficient generation of molecular complexity. The DA reaction is atom-economic and stereospecific, as well as diastereo- and regioselective. The intramolecular version (IMDA) of the DA cycloaddition and its...... and dienophile, methods for acceleration of IMDA reactions (such as use of high pressure) and catalysis (using oxophilic or carbophilic metal complexes, Brønsted acids, and enzymes). The use of furans as diene components (IMDAF), intramolecular hetero-DA (IMHDA) and IMDA reactions with inverse electron demand...... are also covered. Applications of IMDA to asymmetric synthesis (from substrate control through to enantioselective catalysis, including organocatalysis) are presented, along with tandem sequences involving IMDA cycloaddition. A theme pervading the whole chapter is the use of IMDA reactions for the total...

Structure and reactivity of the N-acetyl-cysteine radical cation and anion: does radical migration occur?

NARCIS (Netherlands)

Osburn, S.; Berden, G.; Oomens, J.; O'Hair, R.A.J.; Ryzhov, V.

2011-01-01

The structure and reactivity of the N-acetyl-cysteine radical cation and anion were studied using ion-molecule reactions, infrared multi-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. The radical cation was generated by first nitrosylating the thiol of

Structure and Reactivity of the N-Acetyl-Cysteine Radical Cation and Anion: Does Radical Migration Occur?

NARCIS (Netherlands)

Osburn, S.; G. Berden,; Oomens, J.; O' Hair, R. A. J.; Ryzhov, V.

2011-01-01

The structure and reactivity of the N-acetyl-cysteine radical cation and anion were studied using ion-molecule reactions, infrared multi-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. The radical cation was generated by first nitrosylating the thiol of

Specific Function of the Met-Tyr-Trp Adduct Radical and Residues Arg-418 and Asp-137 in the Atypical Catalase Reaction of Catalase-Peroxidase KatG*

Science.gov (United States)

Zhao, Xiangbo; Khajo, Abdelahad; Jarrett, Sanchez; Suarez, Javier; Levitsky, Yan; Burger, Richard M.; Jarzecki, Andrzej A.; Magliozzo, Richard S.

2012-01-01

Catalase activity of the dual-function heme enzyme catalase-peroxidase (KatG) depends on several structural elements, including a unique adduct formed from covalently linked side chains of three conserved amino acids (Met-255, Tyr-229, and Trp-107, Mycobacterium tuberculosis KatG numbering) (MYW). Mutagenesis, electron paramagnetic resonance, and optical stopped-flow experiments, along with calculations using density functional theory (DFT) methods revealed the basis of the requirement for a radical on the MYW-adduct, for oxyferrous heme, and for conserved residues Arg-418 and Asp-137 in the rapid catalase reaction. The participation of an oxyferrous heme intermediate (dioxyheme) throughout the pH range of catalase activity is suggested from our finding that carbon monoxide inhibits the activity at both acidic and alkaline pH. In the presence of H2O2, the MYW-adduct radical is formed normally in KatG[D137S] but this mutant is defective in forming dioxyheme and lacks catalase activity. KatG[R418L] is also catalase deficient but exhibits normal formation of the adduct radical and dioxyheme. Both mutants exhibit a coincidence between MYW-adduct radical persistence and H2O2 consumption as a function of time, and enhanced subunit oligomerization during turnover, suggesting that the two mutations disrupting catalase turnover allow increased migration of the MYW-adduct radical to protein surface residues. DFT calculations showed that an interaction between the side chain of residue Arg-418 and Tyr-229 in the MYW-adduct radical favors reaction of the radical with the adjacent dioxyheme intermediate present throughout turnover in WT KatG. Release of molecular oxygen and regeneration of resting enzyme are thereby catalyzed in the last step of a proposed catalase reaction. PMID:22918833

Radical's view of sciences